The origins and evolution of sleep

Population dependent rearing modifies sleep and decision-making ability with the involvement of noradrenaline: A study conducted using zebrafish as a model

Sleep is an instinct phenomenon, which affects cognitive processes including learning, memory and decision-making. Its quality, quantity and pattern vary in species through evolution. Living in groups (compared to living individually) with an increased population (as a family and a society) is an important contributing factor influencing the evolution of many behaviours. We hypothesized that rearing (upbringing) among many individuals (as compared to in isolation) and socialization might have influenced the quality, quantity and pattern of optimum sleep (a fundamental behaviour), which in turn affected animal behaviour(s) including learning and decision-making. To confirm, using zebrafish (which expresses shoaling) as a model we evaluated their sleep pattern as well as decision-making ability when reared post-birth under isolated (individually) or populated conditions. We observed that zebrafish reared under isolation affected sleep and compromised their decision-making ability when exposed to predator, which otherwise threatens their survivability. Also, the adverse effects of isolation were improved when the zebrafish were either reared in shoal or, by α1-adrenoceptor antagonist, prazosin, suggesting the role of noradrenaline in mediating the responses. Based on our findings we propose that one of the physiological benefits of living in a society is better (optimum) sleep health, which in turn helps with quality living.

Neuronal correlates of sleep in honey bees

Honey bees Apis mellifera follow the day-night cycle for their foraging activity, entering rest periods during darkness. Despite considerable research on sleep behaviour in bees, its underlying neurophysiological mechanisms are not well understood, partly due to the lack of brain imaging data that allow for analysis from a network- or system-level perspective. This study aims to fill this gap by investigating whether neuronal activity during rest periods exhibits stereotypic patterns comparable to sleep signatures observed in vertebrates. Using two-photon calcium imaging of the antennal lobes (AL) in head-fixed bees, we analysed brain dynamics across motion and rest epochs during the nocturnal period. The recorded activity was computationally characterised, and machine learning was applied to determine whether a classifier could distinguish the two states after motion correction. Out-of-sample classification accuracy reached 93 %, and a feature importance analysis suggested network features to be decisive. Accordingly, the glomerular connectivity was found to be significantly increased in the rest-state patterns. A full simulation of the AL using a leaky spiking neural network revealed that such a transition in network connectivity could be achieved by weakly correlated input noise and a reduction of synaptic conductance of the inhibitive local neurons (LNs) which couple the AL network nodes. The difference in the AL response maps between awake- and sleep-like states generated by the simulation showed a decreased specificity of the odour code in the sleep state, suggesting reduced information processing during sleep. Since LNs in the bee brain are GABAergic, this suggests that the GABAergic system plays a central role in sleep regulation in bees as in many higher species including humans. Our findings support the theoretical view that sleep-related network modulation mechanisms are conserved throughout evolution, highlighting the bee's potential as an invertebrate model for studying sleep at the level of single neurons.

Does It Matter What Keeps You Awake? Effects of Two Different Sleep Deprivation Methods on Object-Location Memory and Hippocampal c-Fos Expression in Mice

In sleep research, various sleep deprivation methods have been used to examine the effects of sleep loss on memory. However, studies often overlook the distinct impacts each method may have on activity in specific neuronal circuits and memory storage. It remains unclear whether these changes following sleep deprivation result from extended wakefulness alone or from an interaction with the nature of the waking experience. To address this question, we examined how two commonly used sleep deprivation methods in mice-gentle handling and novelty-induced sleep deprivation-affect object-location memory and hippocampal c-Fos expression. Using either method, mice were sleep deprived for 3 or 6 h immediately after training in the object-location memory task, and spatial memory performance was assessed 1 day after training. Object-location memory was impaired after 3 and 6 h of novelty-induced sleep deprivation, but only after 6 h of sleep deprivation by gentle handling. Assessing c-Fos expression in separate groups of mice immediately after 3 or 6 h of sleep deprivation showed that both methods increased c-Fos expression in the CA1 and CA3 regions after 3 h of sleep deprivation, while effects in the dentate gyrus depended on the method and blade examined. After 6 h of sleep deprivation, no significant changes in hippocampal c-Fos expression were observed regardless of the method used. Overall, our findings show that the type of experience mice have while being kept awake and the duration of sleep deprivation can have different effects on spatial memory and neuronal activity in hippocampal subregions.

Slumber under pressure: REM sleep and stress response

Sleep, a state of reduced responsiveness and distinct brain activity, is crucial across the animal kingdom. This review explores the potential adaptive functions of REM sleep in adapting to stress, emphasizing its role in memory consolidation, emotional regulation, and threat processing. We further explore the underlying neural mechanisms linking stress responses to REM sleep. By synthesizing current findings, we propose that REM sleep allows animals to "rehearse" or simulate responses to danger in a secure, offline state, while also maintaining emotional balance. Environmental factors, such as predation risk and social dynamics, further influence REM sleep. This modulation may enhance survival by optimizing stress responses while fulfilling physiological needs in animals. Insights into REM sleep's role in animals may shed light on human sleep in the context of modern stressors and sleep disruptions. This review also explores the complex interplay between stress, immunity, sleep disruptions-particularly involving REM sleep-and their evolutionary underpinnings.

The influence of sleep disruption on learning and memory in fish

Sleep is a ubiquitous process that has been conserved in animals. Yet, our understanding of the functions of sleep largely derives from a few species. Sleep is considered to play an important role in mental processes, including learning and memory consolidation, but how widespread this relationship is across taxa remains unclear. Here, we test the impact of sleep disruption on the ability of the cleaner fish (Labroides dimidiatus) to both learn and remember a novel cognitive task. Sleep was disrupted by exposing a subset of fish to light at set intervals during the night. We found a significant negative relationship between sleep disruption and the ability to learn a novel task. Specifically, we found that fish in the light-disturbed sleep treatment took significantly longer and made more incorrect decisions to find a food reward, compared with the undisturbed sleep treatment. All fish were then allowed a normal sleep schedule and retested several days later to assess their ability to remember the task. In contrast to the learning phase, we observed no significant differences between the two treatment groups in remembering the food reward several days later. Our results demonstrate a negative impact of sleep disruption on performance in a cognitive challenging task that appeared to have the strongest effect when fish were first exposed to the challenge. Importantly, we show that the association between sleep and mental processes, such as learning, may be widespread across vertebrate taxa and potentially have an early origin in the evolutionary history of vertebrate animals.

Sleep is necessary for experience-dependent sequence plasticity in mouse primary visual cortex

STUDY OBJECTIVES

Repeated exposure to familiar visual sequences drives experience-dependent and sequence-specific plasticity in mouse primary visual cortex (V1). Prior work demonstrated a critical role for sleep in consolidating a related but mechanistically distinct form of experience-dependent plasticity in V1. Here, we assessed the role of sleep in consolidation of spatiotemporal sequence learning (sequence plasticity) in mouse V1.

METHODS

Visually evoked potentials were recorded in awake, head-fixed mice viewing sequences of four visual stimuli. Each sequence was presented 200 times per session, across multiple sessions, to drive plasticity. The effects of sleep consolidation time and sleep deprivation on plasticity were assessed.

RESULTS

Sequence plasticity occurred in V1 following as little as 1 hour of ad libitum sleep and increased with longer periods of sleep. Sleep deprivation blocked sequence plasticity consolidation, which recovered following subsequent sleep.

CONCLUSIONS

Sleep is required for the consolidation of sequence plasticity in mouse V1.

Daily activity rhythms, sleep, and pregnancy are fundamentally related in the Pacific beetle mimic cockroach, Diploptera punctata

Sleep and pregnancy are contentious bedfellows; sleep disorders and disturbances are associated with adverse pregnancy outcomes, although much is still unknown about this relationship. Sleep and pregnancy have been studied in many models, but most focus heavily on mammals. However, pregnancy is ubiquitous across the animal kingdom - a hallmark of convergent evolution; similarly sleep is a shared feature across diverse species. Here, we present an ideal model in which to study the dynamics between sleep and pregnancy in invertebrates. The Pacific beetle mimic cockroach, Diploptera punctata, is a viviparous cockroach species that uses milk proteins to nourish its young with a broodsac over a three month pregnancy. However, little is known about the relationship between this unique reproductive biology and daily rhythms of activity and sleep. We established that D. punctata displayed a peak in activity shortly following sunset, with males significantly more active than females. When scavenging behavior was examined, males and non-pregnant females emerged more often and traveled further from a shelter compared to pregnant females, suggesting reduced risk-taking behavior in late pregnancy. Chronic disturbance of sleep during pregnancy negatively impacted embryo development by increasing gestational duration and decreasing the transcription of milk proteins. These findings indicate that sleep is key to embryo development and that pregnancy has a significant impact on the daily rhythms of activity in Diploptera punctata. More broadly, we present a tractable invertebrate model for understanding the relationship between sleep and pregnancy, which will aid in the exploration of the poorly understood interface between these two ubiquitous and highly conserved traits.

Supplementation of essential amino acids suppresses age-associated sleep loss and sleep fragmentation but not loss of rhythm strength under yeast-restricted malnutrition in Drosophila

Sleep quality and quantity decrease with age, and sleep disturbance increases the risk of many age-associated diseases. There is a significant relationship between nutritional status and sleep outcomes, with malnutrition inducing poor sleep quality in older adults. However, it remains elusive whether, and if so how, nutritional supplementation prevents age-associated sleep problems. Here, we utilized Drosophila to investigate the effects of a malnutrition diet with restricted yeast, a primary protein source, and supplementation of 10 essential amino acids (EAAs) on sleep profiles during ageing. Compared with the standard diet containing 2.7% yeast, the malnutrition diet containing 0.27% yeast significantly decreased target of rapamycin (TOR) signalling and shortened the lifespan of male Canton-S flies. By contrast, age-associated sleep loss, sleep fragmentation and loss of rhythm strength were similarly observed under both diets. Supplementation of the malnutrition diet with EAAs in restricted yeast significantly ameliorated age-associated sleep loss and sleep fragmentation without altering loss of rhythm strength. It also rescued decreased TOR signalling activity but not the shortened lifespan, suggesting that the effects of EAAs on sleep integrity are independent of TOR activity and lifespan regulation. These results may help to develop dietary interventions that improve age-related sleep problems in humans.

Evolution of canonical circadian clock genes underlies unique sleep strategies of marine mammals for secondary aquatic adaptation

To satisfy the needs of sleeping underwater, marine mammals, including cetaceans, sirenians, and pinnipeds, have evolved an unusual form of sleep, known as unihemispheric slow-wave sleep (USWS), in which one brain hemisphere is asleep while the other is awake. All aquatic cetaceans have only evolved USWS without rapid eye movement (REM) sleep, whereas aquatic sirenians and amphibious pinnipeds display both bihemispheric slow-wave sleep (BSWS) and USWS, as well as REM sleep. However, the molecular genetic changes underlying USWS remain unknown. The present study investigated the evolution of eight canonical circadian genes and found that positive selection occurred mainly within cetacean lineages. Furthermore, convergent evolution was observed in lineages with USWS at three circadian clock genes. Remarkably, in vitro assays showed that cetacean-specific mutations increased the nuclear localization of zebrafish clocka, and enhanced the transcriptional activation activity of Clocka and Bmal1a. In vivo, transcriptome analysis showed that the overexpression of the cetacean-specific mutant clocka (clocka-mut) caused the upregulation of the wakefulness-promoting glutamatergic genes and the differential expression of multiple genes associated with sleep regulation. In contrast, the GABAergic and cholinergic pathways, which play important roles in promoting sleep, were downregulated in the bmal1a-mut-overexpressing zebrafish. Concordantly, sleep time of zebrafish overexpressing clocka-mut and bmal1a-mut were significantly less than the zebrafish overexpressing the wild-type genes, respectively. These findings support our hypothesis that canonical circadian clock genes may have evolved adaptively to enhance circadian regulation ability relating to sleep in cetaceans and, in turn, contribute to the formation of USWS.

Leger RJ. Latitudinal Clines in Climate and Sleep Patterns Shape Disease Outcomes in Drosophila melanogaster Infected by Metarhizium anisopliae

Major latitudinal clines have been observed in Drosophila melanogaster, a human commensal that originated in tropical Africa and has subsequently dispersed globally to colonize temperate habitats. However, despite the crucial role pathogens play in species distribution, our understanding of how geographical factors influence disease susceptibility remains limited. This investigation explored the effects of latitudinal clines and biomes on disease resistance using the common fly pathogen Metarhizium anisopliae and 43 global Drosophila melanogaster populations. The findings revealed correlations between disease resistance and latitudinal gradients of sleep duration, temperature, and humidity. Although enhanced defenses may be driven by fungal diversity at tropical latitudes, the most disease-resistant tropical males also showed the highest susceptibility to desiccation. This suggests potential trade-offs between abiotic stress resistance, necessary for survival in temperate habitats, and disease resistance. Furthermore, the study uncovered interactions between sex, mating status, sleep, and abiotic stresses, affecting disease resistance. Notably, longer-sleeping males and virgin flies survived infections longer, with additional daytime sleep post-infection being protective, particularly in the most resistant fly lines. These observations support the hypothesis that sleep and disease defense are intertwined traits linked to organismal fitness and subject to joint clinal evolution.

The dorsal fan-shaped body is a neurochemically heterogeneous sleep-regulating center in Drosophila

Sleep is a behavior that is conserved throughout the animal kingdom. Yet, despite extensive studies in humans and animal models, the exact function or functions of sleep remain(s) unknown. A complicating factor in trying to elucidate the function of sleep is the complexity and multiplicity of neuronal circuits that are involved in sleep regulation. It is conceivable that distinct sleep-regulating circuits are only involved in specific aspects of sleep and may underlie different sleep functions. Thus, it would be beneficial to assess the contribution of individual circuits in sleep's putative functions. The intricacy of the mammalian brain makes this task extremely difficult. However, the fruit fly Drosophila melanogaster, with its simpler brain organization, available connectomics, and unparalleled genetics, offers the opportunity to interrogate individual sleep-regulating centers. In Drosophila, neurons projecting to the dorsal fan-shaped body (dFB) have been proposed to be key regulators of sleep, particularly sleep homeostasis. We recently demonstrated that the most widely used genetic tool to manipulate dFB neurons, the 23E10-GAL4 driver, expresses in 2 sleep-regulating neurons (VNC-SP neurons) located in the ventral nerve cord (VNC), the fly analog of the vertebrate spinal cord. Since most data supporting a role for the dFB in sleep regulation have been obtained using 23E10-GAL4, it is unclear whether the sleep phenotypes reported in these studies are caused by dFB neurons or VNC-SP cells. A recent publication replicated our finding that 23E10-GAL4 contains sleep-promoting neurons in the VNC. However, it also proposed that the dFB is not involved in sleep regulation at all, but this suggestion was made using genetic tools that are not dFB-specific and a very mild sleep deprivation protocol. In this study, using a newly created dFB-specific genetic driver line, we demonstrate that optogenetic activation of the majority of 23E10-GAL4 dFB neurons promotes sleep and that these neurons are involved in sleep homeostasis. We also show that dFB neurons require stronger stimulation than VNC-SP cells to promote sleep. In addition, we demonstrate that dFB-induced sleep can consolidate short-term memory (STM) into long-term memory (LTM), suggesting that the benefit of sleep on memory is not circuit-specific. Finally, we show that dFB neurons are neurochemically heterogeneous and can be divided in 3 populations. Most dFB neurons express both glutamate and acetylcholine, while a minority of cells expresses only one of these 2 neurotransmitters. Importantly, dFB neurons do not express GABA, as previously suggested. Using neurotransmitter-specific dFB tools, our data also points at cholinergic dFB neurons as particularly potent at regulating sleep and sleep homeostasis.

The subcortical correlates of self-reported sleep quality

Study objectives

To assess the association between self-reported measures of sleep quality and cortical and subcortical local morphometry.

Methods

Sleep quality, operationalized with the Pittsburgh Sleep Quality Index (PSQI), and neuroanatomical data from the full release of the young adult Human Connectome Project dataset were analyzed (N=1,112; 46% female; mean age: 28.8 years old). Local cortical and subcortical morphometry was measured with subject-specific segmentations resulting in voxelwise gray matter difference (i.e., voxel based morephometry) measurements for cortex and local shape measurements for subcortical regions. Associations between the total score of PSQI, two statistical groupings of its subcomponents (obtained with a principal component analysis), and their interaction with demographic (i.e., sex, age, handedness, years of education) and biometric (i.e., BMI) variables were assessed using a general linear model and a nonparametric permutation approach.

Results

Sleep quality-related variance was significantly associated with subcortical morphometry, particularly in the bilateral caudate, putamen, and left pallidum, where smaller shape measures correlated with worse sleep quality. Notably, these associations were independent of demographic and biometric factors. In contrast, cortical morphometry, along with additional subcortical sites, showed no direct associations with sleep quality but demonstrated interactions with demographic and biometric variables.

Conclusions

This study reveals a specific link between self-reported sleep quality and subcortical morphometry, particularly within the striatum and pallidum, reinforcing the role of these regions in sleep regulation. These findings underscore the importance of considering subcortical morphology in sleep research and highlight potential neuromodulatory targets for sleep-related interventions.

The Roles of Discrete Populations of Neurons Expressing Short Neuropeptide F in Sleep Induction in Drosophila melanogaster

Sleep is of vital importance in our lives, yet we are far from understanding the neuronal networks that control the amount and timing of sleep. There is substantial conservation of known sleep-regulating transmitters, allowing for studies in simpler organisms to lead the way in gaining insight into the organization of sleep control circuits. In Drosophila melanogaster, we recently showed that optogenetic activation of neurons that produce the neuropeptide Y (NPY)-related transmitter short neuropeptide F (sNPF) increases time spent asleep. However, sNPF is expressed in several neuronal populations, and thus it is unknown which of those populations play roles in the sleep-promoting effect. In this study, we addressed this issue using a genetic approach to limit optogenetic activation to subsets of sNPF-expressing neurons. We found that sleep promotion was shorter-lived when cryptochrome (CRY)-positive neurons were excluded from being activated. Pigment-dispersing factor (PDF) neurons were not required for sleep promotion, nor were mushroom body (MB) neurons. Acute reactions to a short, 10-s period of optogenetic activation were largely unchanged by excluding activation of the three neuronal populations mentioned above. Together, these results suggest that clock neurons that are CRY-positive and PDF-negative are important contributors to the long-lasting sleep promotion produced by sNPF neuron activation. However, other neurons targeted by the sNPF-GAL4 driver appear to mediate the more rapid behavioral responses. Future studies will seek to identify these additional sNPF neuron populations and to determine how sNPF-expressing clock neurons act in concert with other neuronal circuits to promote sleep.

Calcineurin: An essential regulator of sleep revealed by biochemical, chemical biological, and genetic approaches

Research into mechanisms underlying sleep traditionally relies on electrophysiology and genetics. Because sleep can only be measured on whole animals by behavioral observations and physical means, no sleep research was initiated by biochemical and chemical biological approaches. We used phosphorylation sites of kinases important for sleep as targets for biochemical and chemical biological approaches. Sleep was increased in mice carrying a threonine-to-alanine substitution at residue T469 of salt-inducible kinase 3 (SIK3). Our biochemical purification and photo-crosslinking revealed calcineurin (CaN) dephosphorylation, both in vitro and in vivo, of SIK3 at T469 and S551, but not T221. Knocking down CaN regulatory subunit reduced daily sleep by more than 5 h, exceeding all known mouse mutants. Our work uncovered a critical physiological role for CaN in sleep and pioneered biochemical purification and chemical biology as effective approaches to study sleep.

Decoding the Functional Interactome of Non-Model Organisms with PHILHARMONIC

Protein-protein interaction (PPI) networks are a fundamental resource for modeling cellular and molecular function, and a large and sophisticated toolbox has been developed to leverage their structure and topological organization to predict the functional roles of under-studied genes, proteins, and pathways. However, the overwhelming majority of experimentally-determined interactions from which such networks are constructed come from a small number of well-studied model organisms. Indeed, most species lack even a single experimentally-determined interaction in these databases, much less a network to enable the analysis of cellular function, and methods for computational PPI prediction are too noisy to apply directly. We introduce PHILHARMONIC, a novel computational approach that couples deep learning de novo network inference with robust unsupervised spectral clustering algorithms to uncover functional relationships and high-level organization in non-model organisms. Our clustering approach allows us to de-noise the predicted network, producing highly informative functional modules. We also develop a novel algorithm called ReCIPE, which aims to reconnect disconnected clusters, increasing functional enrichment and biological interpretability. We perform remote homology-based functional annotation by leveraging hmmscan and GODomainMiner to assign initial functions to proteins at large evolutionary distances. Our clusters enable us to newly assign functions to uncharacterized proteins through "function by association." We demonstrate the ability of PHILHARMONIC to recover clusters with significant functional coherence in the reef-building coral P. damicornis, its algal symbiont C. goreaui, and the well-annotated fruit fly D. melanogaster. We perform a deeper analysis of the P. damicornis network, where we show that PHILHARMONIC clusters correlate strongly with gene co-expression and investigate several clusters that participate in temperature regulation in the coral, including the first putative functional annotation of several previously uncharacterized proteins. Easy to run end-to-end and requiring only a sequenced proteome, PHILHARMONIC is an engine for biological hypothesis generation and discovery in non-model organisms.

Investigating the consequences of chronic short sleep for metabolism and survival of oxidative stress

In previous work, we found that short sleep caused sensitivity to oxidative stress; here we set out to characterize the physiological state of a diverse group of chronically short-sleeping mutants during hyperoxia as an acute oxidative stress. Using RNA-sequencing analysis, we found that short-sleeping mutants had a normal transcriptional oxidative stress response relative to controls. In both short-sleeping mutants and controls, hyperoxia led to downregulation of glycolytic genes and upregulation of genes involved in fatty acid metabolism, reminiscent of metabolic shifts during sleep. We hypothesized that short-sleeping mutants may be sensitive to hyperoxia because of defects in metabolism. Consistent with this, short-sleeping mutants were sensitive to starvation. Using metabolomics, we identified a pattern of low levels of long chain fatty acids and lysophospholipids in short-sleeping mutants relative to controls during hyperoxia, suggesting a defect in lipid metabolism. Though short-sleeping mutants did not have common defects in many aspects of lipid metabolism (basal fat stores, usage kinetics during hyperoxia, respiration rates, and cuticular hydrocarbon profiles), they were all sensitive to dehydration, suggesting a general defect in cuticular hydrocarbons, which protect against dehydration. To test the bi-directionality of sleep and lipid metabolism, we tested flies with both diet-induced obesity and genetic obesity. Flies with diet-induced obesity had no sleep or oxidative stress phenotype; in contrast, the lipid metabolic mutant, brummer , slept significantly more than controls but was sensitive to oxidative stress. Previously, all short sleepers tested were sensitive and all long sleepers resistant to oxidative stress. brummer mutants, the first exceptions to this rule, lack a key enzyme required to mobilize fat stores, suggesting that a defect in accessing lipid stores can cause sensitivity to oxidative stress. Taken together, we found that short-sleeping mutants have many phenotypes in common: sensitivity to oxidative stress, starvation, dehydration, and defects in lipid metabolites. These results argue against a specific role for sleep as an antioxidant and suggest the possibility that lipid metabolic defects underlie the sensitivity to oxidative stress of short-sleeping mutants.

Postprandial Sleep in Short-Sleeping Mexican Cavefish

Interactions between sleep and feeding behaviors are critical for adaptive fitness. Diverse species suppress sleep when food is scarce to increase the time spent foraging. Postprandial sleep, an increase in sleep time following a feeding event, has been documented in vertebrate and invertebrate animals. While interactions between sleep and feeding appear to be highly conserved, the evolution of postprandial sleep in response to changes in food availability remains poorly understood. Multiple populations of the Mexican cavefish, Astyanax mexicanus, have independently evolved sleep loss and increased food consumption compared to surface-dwelling fish of the same species, providing the opportunity to investigate the evolution of interactions between sleep and feeding. Here, we investigate the effects of feeding on sleep in larval and adult surface fish, and in two parallelly evolved cave populations of A. mexicanus. Larval surface and cave populations of A. mexicanus increase sleep immediately following a meal, providing the first evidence of postprandial sleep in a fish model. The amount of sleep was not correlated to meal size and occurred independently of feeding time. In contrast to larvae, postprandial sleep was not detected in adult surface or cavefish, which can survive for months without food. Together, these findings reveal that postprandial sleep is present in multiple short-sleeping populations of cavefish, suggesting sleep-feeding interactions are retained despite the evolution of sleep loss. These findings raise the possibility that postprandial sleep is critical for energy conservation and survival in larvae that are highly sensitive to food deprivation.

Stem cell-specific ecdysone signaling regulates the development of dorsal fan-shaped body neurons and sleep homeostasis

Complex behaviors arise from neural circuits that assemble from diverse cell types. Sleep is a conserved behavior essential for survival, yet little is known about how the nervous system generates neuron types of a sleep-wake circuit. Here, we focus on the specification of Drosophila 23E10-labeled dorsal fan-shaped body (dFB) long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex. We use lineage analysis and genetic birth dating to identify two bilateral type II neural stem cells (NSCs) that generate 23E10 dFB neurons. We show that adult 23E10 dFB neurons express ecdysone-induced protein 93 (E93) and that loss of ecdysone signaling or E93 in type II NSCs results in their misspecification. Finally, we show that E93 knockdown in type II NSCs impairs adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate the neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.

Elevated DNA Damage without signs of aging in the short-sleeping Mexican Cavefish

Dysregulation of sleep has widespread health consequences and represents an enormous health burden. Short-sleeping individuals are predisposed to the effects of neurodegeneration, suggesting a critical role for sleep in the maintenance of neuronal health. While the effects of sleep on cellular function are not completely understood, growing evidence has identified an association between sleep loss and DNA damage, raising the possibility that sleep facilitates efficient DNA repair. The Mexican tetra fish, Astyanax mexicanus provides a model to investigate the evolutionary basis for changes in sleep and the consequences of sleep loss. Multiple cave-adapted populations of these fish have evolved to sleep for substantially less time compared to surface populations of the same species without identifiable impacts on healthspan or longevity. To investigate whether the evolved sleep loss is associated with DNA damage and cellular stress, we compared the DNA Damage Response (DDR) and oxidative stress levels between A. mexicanus populations. We measured markers of chronic sleep loss and discovered elevated levels of the DNA damage marker γH2AX in the brain, and increased oxidative stress in the gut of cavefish, consistent with chronic sleep deprivation. Notably, we found that acute UV-induced DNA damage elicited an increase in sleep in surface fish but not in cavefish. On a transcriptional level, only the surface fish activated the photoreactivation repair pathway following UV damage. These findings suggest a reduction of the DDR in cavefish compared to surface fish that coincides with elevated DNA damage in cavefish. To examine DDR pathways at a cellular level, we created an embryonic fibroblast cell line from the two populations of A. mexicanus. We observed that both the DDR and DNA repair were diminished in the cavefish cells, corroborating the in vivo findings and suggesting that the acute response to DNA damage is lost in cavefish. To investigate the long-term impact of these changes, we compared the transcriptome in the brain and gut of aged surface fish and cavefish. Strikingly, many genes that are differentially expressed between young and old surface fish do not transcriptionally vary by age in cavefish. Taken together, these findings suggest that cavefish have developed resilience to sleep loss, despite possessing cellular hallmarks of chronic sleep deprivation.

Tapeworm infection affects sleep-like behavior in three-spined sticklebacks

Sleep is a complex and conserved biological process that affects several body functions and behaviors. Evidence suggests that there is a reciprocal interaction between sleep and immunity. For instance, fragmented sleep can increase the probability of parasitic infections and reduce the ability to fight infections. Moreover, viral and bacterial infections alter the sleep patterns of infected individuals. However, the effects of macro-parasitic infections on sleep remain largely unknown, and measuring sleep in non-model organisms remains challenging. In this study, we investigated whether macro-parasite infections could alter sleep-like behavior of their hosts. We experimentally infected three-spined sticklebacks (Gasterosteus aculeatus), a freshwater fish, with the tapeworm Schistocephalus solidus and used a hidden Markov model to characterize sleep-like behavior in sticklebacks. One to four days after parasite exposure, infected fish showed no difference in sleep-like behavior compared with non-exposed fish, and fish that were exposed-but-not-infected only showed a slight reduction in sleep-like behavior during daytime. Twenty-nine to 32 days after exposure, infected fish showed more sleep-like behavior than control fish, while exposed-but-not-infected fish showed overall less sleep-like behavior. Using brain transcriptomics, we identified immune- and sleep-associated genes that potentially underlie the observed behavioral changes. These results provide insights into the complex association between macro-parasite infection, immunity, and sleep in fish and may thus contribute to a better understanding of reciprocal interactions between sleep and immunity.

Alzheimer's disease and sleep disorders: A bidirectional relationship

Alzheimer's disease (AD) is the most prevalent dementia, pathologically featuring abnormal accumulation of amyloid-β (Aβ) and hyperphosphorylated tau, while sleep, divided into rapid eye movement sleep (REM) and nonrapid eye movement sleep (NREM), plays a key role in consolidating social and spatial memory. Emerging evidence has revealed that sleep disorders such as circadian disturbances and disruption of neuronal rhythm activity are considered as both candidate risks and consequence of AD, suggesting a bidirectional relationship between sleep and AD. This review will firstly grasp basic knowledge of AD pathogenesis, then highlight macrostructural and microstructural alteration of sleep along with AD progression, explain the interaction between accumulation of Aβ and hyperphosphorylated tau, which are two critical neuropathological processes of AD, as well as neuroinflammation and sleep, and finally introduce several methods of sleep enhancement as strategies to reduce AD-associated neuropathology. Although theories about the bidirectional relationship and relevant therapeutic methods in mice have been well developed in recent years, the knowledge in human is still limited. More studies on how to effectively ameliorate AD pathology in patients by sleep enhancement and what specific roles of sleep play in AD are needed.

Whole-Brain Electrophysiology and Calcium Imaging in Drosophila during Sleep and Wake

Sleep is likely a whole-brain phenomenon, with most of the brain probably benefiting from this state of decreased arousal. Recent advances in our understanding of some potential sleep functions, such as metabolite clearance and synaptic homeostasis, make it evident why the whole brain is likely impacted by sleep: All neurons have synapses, and all neurons produce waste metabolites. Sleep experiments in the fly Drosophila melanogaster suggest that diverse sleep functions appear to be conserved across all animals. Studies of brain activity during sleep in humans typically involve multidimensional data sets, such as those acquired by electroencephalograms (EEGs) or functional magnetic resonance imaging (fMRI), and these whole-brain read-outs often reveal important qualities of different sleep stages, such as changes in frequency dynamics or connectivity. Recently, various techniques have been developed that allow for the recording of neural activity simultaneously across multiple regions of the fly brain. These whole-brain-recording approaches will be important for better understanding sleep physiology and function, as they provide a more comprehensive view of neural dynamics during sleep and wake in a relevant model system. Here, we present a brief summary of some of the findings derived from sleep activity recording studies in sleeping Drosophila flies and discuss the value of electrophysiological versus calcium imaging techniques. Although these involve very different preparations, they both highlight the value of multidimensional data for studying sleep in this model system, like the use of both EEG and fMRI in humans.

Methods used for estimating sleep in dairy cattle

Sleep serves several essential functions in all mammals including dairy cattle. Researchers are beginning to estimate sleep in dairy cattle using a combination of physiological measurements (e.g., polysomnography) as well as changes in behavior (e.g., different resting postures). Sleep may provide unique insight into how cows and calves respond to, and cope with, their environments, as a complement to other common measurements such as lying time. Although each of the methods to assess sleep in cattle has its advantages, there remain several challenges with each approach. The objective of this narrative mini-review is to describe current methods for estimating sleep in dairy cattle, including some of the advantages and limitations with each method. We will start with describing the research to date on adult cows, followed by preweaning dairy calves. We end the review with recommendations for researchers interested in assessing sleep in dairy cattle and ideas for future areas of research.

Postprandial sleep in short-sleeping Mexican cavefish

Interaction between sleep and feeding behaviors are critical for adaptive fitness. Diverse species suppress sleep when food is scarce to increase the time spent foraging. Post-prandial sleep, an increase in sleep time following a feeding event, has been documented in vertebrate and invertebrate animals. While interactions between sleep and feeding appear to be highly conserved, the evolution of postprandial sleep in response to changes in food availability remains poorly understood. Multiple populations of the Mexican cavefish, Astyanax mexicanus, have independently evolved sleep loss and increased food consumption compared to surface-dwelling fish of the same species, providing the opportunity to investigate the evolution of interactions between sleep and feeding. Here, we investigate effects of feeding on sleep in larval and adult surface fish, and two parallelly evolved cave populations of A. mexicanus. Larval surface and cave populations of A. mexicanus increase sleep immediately following a meal, providing the first evidence of postprandial sleep in a fish model. The amount of sleep was not correlated to meal size and occurred independently of feeding time. In contrast to larvae, postprandial sleep was not detected in adult surface or cavefish, that can survive for months without food. Together, these findings reveal that postprandial sleep is present in multiple short-sleeping populations of cavefish, suggesting sleep-feeding interactions are retained despite the evolution of sleep loss. These findings raise the possibility that postprandial sleep is critical for energy conservation and survival in larvae that are highly sensitive to food deprivation.

The Complex Relationship between Sleep and Cognitive Reserve: A Narrative Review Based on Human Studies

Sleep and brain/cognitive/neural reserve significantly impact well-being and cognition throughout life. This review aims to explore the intricate relationship between such factors, with reference to their effects on human cognitive functions. The specific goal is to understand the bidirectional influence that sleep and reserve exert on each other. Up to 6 February 2024, a methodical search of the literature was conducted using the PubMed database with terms related to brain, cognitive or neural reserve, and healthy or disturbed sleep. Based on the inclusion criteria, 11 articles were selected and analyzed for this review. The articles focus almost exclusively on cognitive reserve, with no explicit connection between sleep and brain or neural reserve. The results evidence sleep's role as a builder of cognitive reserve and cognitive reserve's role as a moderator in the effects of physiological and pathological sleep on cognitive functions. In conclusion, the findings of the present review support the notion that both sleep and cognitive reserve are critical factors in cognitive functioning. Deepening comprehension of the interactions between them is essential for devising strategies to enhance brain health and resilience against age- and pathology-related conditions.

The Impact of Sleep on Haematological Parameters in Firefighters

Sleep is a vital process that impacts biological functions such as cell renewal, bone regeneration, and immune system support. Disrupted sleep can interrupt erythropoiesis, leading to fewer red blood cells, reduced haemoglobin concentration, and decreased haematocrit levels, potentially contributing to haematological disorders. This is particularly concerning for shift workers for example firefighters. While previous studies have explored sleep's adverse effects on various professions, research specific to firefighters is limited. This study investigates the relationship between sleep quality and haematological parameters among firefighters in Northeast Portugal. From a sample of 201 firefighters, variations in red blood cells, haemoglobin, and haematocrit values were linked to sleep quality. The study utilised non-parametric tests (Wilcoxon-Mann-Whitney, Spearman's correlation) to explore the connection between sleep quality and haematological profile. The impact of covariates on haematological parameters was assessed using non-parametric ANCOVA (Quade's). A multiple regression analysis was employed to further understand how sleep quality and various confounding variables impact haematological levels. Findings suggest a negative link between sleep quality and haematological levels, meaning that as sleep quality deteriorates, there is a tendency for haematological levels to decrease, as indicated by Spearman's correlation (rRBC = -0.157, pRBC = 0.026; rHb = -0.158, pHb = 0.025; rHCT = -0.175, pHCT = 0.013). As observed in scientific literature, the correlation found suggests a possible inhibition of erythropoiesis, the process responsible for red blood cell production. Despite firefighters presenting a haematological profile within the reference range (RBC: 5.1 × 106/mm3 (SD ± 0.4), Hb: 15.6 g/dL (SD ± 1.3), 47% (SD ± 1.0), there is already an observable trend towards lower levels. The analysis of co-variables did not reveal a significant impact of sleep quality on haematological levels. In conclusion, this study underscores the importance of sleep quality in determining haematological parameters among firefighters. Future research should investigate the underlying mechanisms and long-term implications of poor sleep quality on firefighter health. Exploring interventions to enhance sleep quality is vital for evidence-based strategies promoting firefighter well-being.

Measuring Sleep and Activity Patterns in Adult Zebrafish

Sleep is an essential behavior that is still poorly understood. Sleep abnormalities accompany a variety of psychiatric and neurological disorders, and sleep can serve as a modifiable behavior in the treatment of these disorders. Zebrafish (Danio rerio) has proven to be a powerful model organism to study sleep and the interplay between sleep and these disorders due to the high conservation of the neuro-modulatory mechanisms that control sleep and wake states between zebrafish and humans. The zebrafish is a diurnal vertebrate with a relatively simple nervous system compared to mammalian models, exhibiting conservation of sleep ontogeny across different life stages. Zebrafish larvae are an established high-throughput model to assess sleep phenotypes and the biological underpinnings of sleep disturbances. To date, sleep measurement in juvenile and adult zebrafish has not been performed in a standardized and reproducible manner because of the relatively low-throughput nature in relation to their larval counterparts. This has left a gap in understanding sleep across later stages of life that are relevant to many psychiatric and neurodegenerative disorders. Several research groups have used homemade systems to address this gap. Here, we report employing commercially available equipment to track activity and sleep/wake patterns in juvenile and adult zebrafish. The equipment allows researchers to perform automated behavior assays in an isolated environment with light/dark and temperature control for multiple days. We first explain the experimental procedure to track the sleep and activity of adult zebrafish and then validate the protocol by measuring the effects of melatonin and DMSO administration. Key features • Allows an isolated and controllable environment to carry out activity and sleep assays in juvenile and adult zebrafish. • Measures activity of zebrafish in life stages later than early development, which requires feeding animals during the assay. • Requires use of a commercially available equipment system and six tanks. • The activity of zebrafish can be tracked for five days including an acclimation step.

Evo-devo applied to sleep research: an approach whose time has come

Sleep occurs in all animals but its amount, form, and timing vary considerably between species and between individuals. Currently, little is known about the basis for these differences, in part, because we lack a complete understanding of the brain circuitry controlling sleep-wake states and markers for the cell types which can identify similar circuits across phylogeny. Here, I explain the utility of an "Evo-devo" approach for comparative studies of sleep regulation and function as well as for sleep medicine. This approach focuses on the regulation of evolutionary ancient transcription factors which act as master controllers of cell-type specification. Studying these developmental transcription factor cascades can identify novel cell clusters which control sleep and wakefulness, reveal the mechanisms which control differences in sleep timing, amount, and expression, and identify the timepoint in evolution when different sleep-wake control neurons appeared. Spatial transcriptomic studies, which identify cell clusters based on transcription factor expression, will greatly aid this approach. Conserved developmental pathways regulate sleep in mice, Drosophila, and C. elegans. Members of the LIM Homeobox (Lhx) gene family control the specification of sleep and circadian neurons in the forebrain and hypothalamus. Increased Lhx9 activity may account for increased orexin/hypocretin neurons and reduced sleep in Mexican cavefish. Other transcription factor families specify sleep-wake circuits in the brainstem, hypothalamus, and basal forebrain. The expression of transcription factors allows the generation of specific cell types for transplantation approaches. Furthermore, mutations in developmental transcription factors are linked to variation in sleep duration in humans, risk for restless legs syndrome, and sleep-disordered breathing. This paper is part of the "Genetic and other molecular underpinnings of sleep, sleep disorders, and circadian rhythms including translational approaches" collection.

Divergent evolution of sleep in Drosophila species

Living organisms synchronize their biological activities with the earth's rotation through the circadian clock, a molecular mechanism that regulates biology and behavior daily. This synchronization factually maximizes positive activities (e.g., social interactions, feeding) during safe periods, and minimizes exposure to dangers (e.g., predation, darkness) typically at night. Beyond basic circadian regulation, some behaviors like sleep have an additional layer of homeostatic control, ensuring those essential activities are fulfilled. While sleep is predominantly governed by the circadian clock, a secondary homeostatic regulator, though not well-understood, ensures adherence to necessary sleep amounts and hints at a fundamental biological function of sleep beyond simple energy conservation and safety. Here we explore sleep regulation across seven Drosophila species with diverse ecological niches, revealing that while circadian-driven sleep aspects are consistent, homeostatic regulation varies significantly. The findings suggest that in Drosophilids, sleep evolved primarily for circadian purposes. The more complex, homeostatically regulated functions of sleep appear to have evolved independently in a species-specific manner, and are not universally conserved. This laboratory model may reproduce and recapitulate primordial sleep evolution.

Ontogeny and social context regulate the circadian activity patterns of Lake Malawi cichlids

Activity patterns tend to be highly stereotyped and critical for executing many different behaviors including foraging, social interactions, and predator avoidance. Differences in the circadian timing of locomotor activity and rest periods can facilitate habitat partitioning and the exploitation of novel niches. As a consequence, closely related species often display highly divergent activity patterns, suggesting that shifts from diurnal to nocturnal behavior, or vice versa, are critical for survival. In Africa's Lake Malawi alone, there are over 500 species of cichlids, which inhabit diverse environments and exhibit extensive phenotypic variation. We have previously identified a substantial range in activity patterns across adult Lake Malawi cichlid species, from strongly diurnal to strongly nocturnal. In many species, including fishes, ecological pressures differ dramatically across life-history stages, raising the possibility that activity patterns may change over ontogeny. To determine if rest-activity patterns change across life stages, we compared the locomotor patterns of six Lake Malawi cichlid species. While total rest and activity did not change between early juvenile and adult stages, rest-activity patterns did, with juveniles displaying distinct activity rhythms that are more robust than adults. One distinct difference between juveniles and adults is the emergence of complex social behavior. To determine whether social context is required for activity rhythms, we next measured locomotor behavior in group-housed adult fish. We found that when normal social interactions were allowed, locomotor activity patterns were restored, supporting the notion that social interactions promote circadian regulation of activity in adult fish. These findings reveal a previously unidentified link between developmental stage and social interactions in the circadian timing of cichlid activity.

Noradrenergic tone is not required for neuronal activity-induced rebound sleep in zebrafish

Sleep pressure builds during wakefulness, but the mechanisms underlying this homeostatic process are poorly understood. One zebrafish model suggests that sleep pressure increases as a function of global neuronal activity, such as during sleep deprivation or acute exposure to drugs that induce widespread brain activation. Given that the arousal-promoting noradrenergic system is important for maintaining heightened neuronal activity during wakefulness, we hypothesised that genetic and pharmacological reduction of noradrenergic tone during drug-induced neuronal activation would dampen subsequent rebound sleep in zebrafish larvae. During stimulant drug treatment, dampening noradrenergic tone with the α2-adrenoceptor agonist clonidine unexpectedly enhanced subsequent rebound sleep, whereas enhancing noradrenergic signalling with a cocktail of α1- and β-adrenoceptor agonists did not enhance rebound sleep. Similarly, CRISPR/Cas9-mediated elimination of the dopamine β-hydroxylase (dbh) gene, which encodes an enzyme required for noradrenalin synthesis, enhanced baseline sleep in larvae but did not prevent additional rebound sleep following acute induction of neuronal activity. Across all drug conditions, c-fos expression immediately after drug exposure correlated strongly with the amount of induced rebound sleep, but was inversely related to the strength of noradrenergic modulatory tone. These results are consistent with a model in which increases in neuronal activity, as reflected by brain-wide levels of c-fos induction, drive a sleep pressure signal that promotes rebound sleep independently of noradrenergic tone.

The interplay between sleep and ecophysiology, behaviour and responses to environmental change in fish

Evidence of behavioural sleep has been observed in every animal species studied to date, but current knowledge of the behaviour, neurophysiology and ecophysiology associated with sleep is concentrated on mammals and birds. Fish are a hugely diverse group that can offer novel insights into a variety of sleep-related behaviours across environments, but the ecophysiological relevance of sleep in fish has been largely overlooked. Here, we systematically reviewed the literature to assess the current breadth of knowledge on fish sleep, and surveyed the diverse physiological effects and behaviours associated with sleep. We also discuss possible ways in which unstudied external factors may alter sleep behaviours. For example, predation risk may alter sleep patterns, as has been shown in mammalian, avian and reptilian species. Other environmental factors - such as water temperature and oxygen availability - have the potential to alter sleep patterns in fish differently than for terrestrial endotherms. Understanding the ecological influences on sleep in fish is vital, as sleep deprivation has the potential to affect waking behaviour and fitness owing to cognitive and physiological impairments, possibly affecting ecological phenomena and sensitivity to environmental stressors in ways that have not been considered.

Phylogenetic conservation of the interdependent homeostatic relationship of sleep regulation and redox metabolism

Sleep is an essential and evolutionarily conserved process that affects many biological functions that are also strongly regulated by cellular metabolism. The interdependence between sleep homeostasis and redox metabolism, in particular, is such that sleep deprivation causes redox metabolic imbalances in the form of over-production of ROS. Likewise (and vice versa), accumulation of ROS leads to greater sleep pressure. Thus, it is theorized that one of the functions of sleep is to act as the brain's "antioxidant" at night by clearing oxidation built up from daily stress of the active day phase. In this review, we will highlight evidence linking sleep homeostasis and regulation to redox metabolism by discussing (1) the bipartite role that sleep-wake neuropeptides and hormones have in redox metabolism through comparing cross-species cellular and molecular mechanisms, (2) the evolutionarily metabolic changes that accompanied the development of sleep loss in cavefish, and finally, (3) some of the challenges of uncovering the cellular mechanism underpinning how ROS accumulation builds sleep pressure and cellularly, how this pressure is cleared.

Microglia modulate sleep/wakefulness under baseline conditions and under acute social defeat stress in adult mice

Although sleep is tightly regulated by multiple neuronal circuits in the brain, nonneuronal cells such as glial cells have been increasingly recognized as crucial sleep regulators. Recent studies have shown that microglia may act to maintain wakefulness. Here, we investigated the possible involvement of microglia in the regulation of sleep quantity and quality under baseline and stress conditions through electroencephalography (EEG)/electromyography (EMG) recordings, and by employing pharmacological methods to eliminate microglial cells in the adult mouse brain. We found that severe microglial depletion induced by the colony-stimulating factor 1 receptor (CSF1R) antagonist PLX5622 (PLX) reversibly decreased the total wake time and the wake episode duration and increased the EEG slow-wave power during wakefulness under baseline conditions. To examine the role of microglia in sleep/wake regulation under mental stress, we used the acute social defeat stress (ASDS) paradigm, an ethological model for psychosocial stress. Sleep analysis under ASDS revealed that microglial depletion exacerbated the stress-induced decrease in the total wake time and increase in anxiety-like behaviors in the open field test. These results demonstrate that microglia actively modulate sleep quantity and architecture under both baseline and stress conditions. Our findings suggest that microglia may potentially provide resilience against acute psychosocial stress by regulating restorative sleep.

Tob Regulates the Timing of Sleep Onset at Night in Drosophila

Sleep is regulated by homeostatic sleep drive and the circadian clock. While tremendous progress has been made in elucidating the molecular components of the core circadian oscillator, the output mechanisms by which this robust oscillator generates rhythmic sleep behavior remain poorly understood. At the cellular level, growing evidence suggests that subcircuits in the master circadian pacemaker suprachiasmatic nucleus (SCN) in mammals and in the clock network in Drosophila regulate distinct aspects of sleep. Thus, to identify novel molecules regulating the circadian timing of sleep, we conducted a large-scale screen of mouse SCN-enriched genes in Drosophila Here, we show that Tob (Transducer of ERB-B2) regulates the timing of sleep onset at night in female fruit flies. Knockdown of Tob pan-neuronally, either constitutively or conditionally, advances sleep onset at night. We show that Tob is specifically required in "evening neurons" (the LNds and the fifth s-LNv) of the clock network for proper timing of sleep onset. Tob levels cycle in a clock-dependent manner in these neurons. Silencing of these "evening" clock neurons results in an advanced sleep onset at night, similar to that seen with Tob knockdown. Finally, sharp intracellular recordings demonstrate that the amplitude and kinetics of LNd postsynaptic potentials (PSPs) cycle between day and night, and this cycling is attenuated with Tob knockdown in these cells. Our data suggest that Tob acts as a clock output molecule in a subset of clock neurons to potentiate their activity in the evening and enable the proper timing of sleep onset at night.

A vertebrate family without a functional Hypocretin/Orexin arousal system

The Hypocretin/Orexin signaling pathway suppresses sleep and promotes arousal, whereas the loss of Hypocretin/Orexin results in narcolepsy, including the involuntary loss of muscle tone (cataplexy).1 Here, we show that the South Asian fish species Chromobotia macracanthus exhibits a sleep-like state during which individuals stop swimming and rest on their side. Strikingly, we discovered that the Hypocretin/Orexin system is pseudogenized in C. macracanthus, but in contrast to Hypocretin-deficient mammals, C. macracanthus does not suffer from sudden behavioral arrests. Similarly, zebrafish mutations in hypocretin/orexin show no evident signs of cataplectic-like episodes. Notably, four additional species in the Botiidae family also lack a functional Hypocretin/Orexin system. These findings identify the first vertebrate family that does not rely on a functional Hypocretin/Orexin system for the regulation of sleep and arousal.

Individual sleep need is flexible and dynamically related to cognitive function

Given that sleep deprivation studies consistently show that short sleep causes neurocognitive deficits, the effects of insufficient sleep on brain health and cognition are of great interest and concern. Here we argue that experimentally restricted sleep is not a good model for understanding the normal functions of sleep in naturalistic settings. Cross-disciplinary research suggests that human sleep is remarkably dependent on environmental conditions and social norms, thus escaping universally applicable rules. Sleep need varies over time and differs between individuals, showing a complex relationship with neurocognitive function. This aspect of sleep is rarely addressed in experimental work and is not reflected in expert recommendations about sleep duration. We recommend focusing on the role of individual and environmental factors to improve our understanding of the relationship between human sleep and cognition.

Neurophysiological and behavioral synchronization in group-living and sleeping mice

Social interactions profoundly influence animal development, physiology, and behavior. Yet, how sleep-a central behavioral and neurophysiological process-is modulated by social interactions is poorly understood. Here, we characterized sleep behavior and neurophysiology in freely moving and co-living mice under different social conditions. We utilized wireless neurophysiological devices to simultaneously record multiple individuals within a group for 24 h, alongside video acquisition. We first demonstrated that mice seek physical contact before sleep initiation and sleep while in close proximity to each other (hereafter, "huddling"). To determine whether huddling during sleep is a motivated behavior, we devised a novel behavioral apparatus allowing mice to choose whether to sleep in close proximity to a conspecific or in solitude, under different environmental conditions. We also applied a deep-learning-based approach to classify huddling behavior. We demonstrate that mice are willing to forgo their preferred sleep location, even under thermoneutral conditions, to gain access to social contact during sleep. This strongly suggests that the motivation for prolonged physical contact-which we term somatolonging-drives huddling behavior. We then characterized sleep architecture under different social conditions and uncovered a social-dependent modulation of sleep. We also revealed coordination in multiple neurophysiological features among co-sleeping individuals, including in the timing of falling asleep and waking up and non-rapid eye movement sleep (NREMS) intensity. Notably, the timing of rapid eye movement sleep (REMS) was synchronized among co-sleeping male siblings but not co-sleeping female or unfamiliar mice. Our findings provide novel insights into the motivation for physical contact and the extent of social-dependent plasticity in sleep.

Lessons from lonely flies: Molecular and neuronal mechanisms underlying social isolation

Animals respond to changes in the environment which affect their internal state by adapting their behaviors. Social isolation is a form of passive environmental stressor that alters behaviors across animal kingdom, including humans, rodents, and fruit flies. Social isolation is known to increase violence, disrupt sleep and increase depression leading to poor mental and physical health. Recent evidences from several model organisms suggest that social isolation leads to remodeling of the transcriptional and epigenetic landscape which alters behavioral outcomes. In this review, we explore how manipulating social experience of fruit fly Drosophila melanogaster can shed light on molecular and neuronal mechanisms underlying isolation driven behaviors. We discuss the recent advances made using the powerful genetic toolkit and behavioral assays in Drosophila to uncover role of neuromodulators, sensory modalities, pheromones, neuronal circuits and molecular mechanisms in mediating social isolation. The insights gained from these studies could be crucial for developing effective therapeutic interventions in future.

A protocol for whole-brain Ca2+ imaging in Astyanax mexicanus, a model of comparative evolution

In this protocol, we describe a comparative approach to study the evolution of brain function in the Mexican tetra, Astyanax mexicanus. We developed surface fish and two independent populations of cavefish with pan-neuronal expression of the Ca2+ sensor GCaMP6s. We describe a methodology to prepare samples and image activity across the optic tectum and olfactory bulb.

Homeostatic control of deep sleep and molecular correlates of sleep pressure in Drosophila

Homeostatic control of sleep is typically addressed through mechanical stimulation-induced forced wakefulness and the measurement of subsequent increases in sleep. A major confound attends this approach: biological responses to deprivation may reflect a direct response to the mechanical insult rather than to the loss of sleep. Similar confounds accompany all forms of sleep deprivation and represent a major challenge to the field. Here, we describe a new paradigm for sleep deprivation in Drosophila that fully accounts for sleep-independent effects. Our results reveal that deep sleep states are the primary target of homeostatic control and establish the presence of multi-cycle sleep rebound following deprivation. Furthermore, we establish that specific deprivation of deep sleep states results in state-specific homeostatic rebound. Finally, by accounting for the molecular effects of mechanical stimulation during deprivation experiments, we show that serotonin levels track sleep pressure in the fly's central brain. Our results illustrate the critical need to control for sleep-independent effects of deprivation when examining the molecular correlates of sleep pressure and call for a critical reassessment of work that has not accounted for such non-specific effects.

Forward genetic screen of Caenorhabditis elegans mutants with impaired sleep reveals a crucial role of neuronal diacylglycerol kinase DGK-1 in regulating sleep

The sleep state is widely observed in animals. The molecular mechanisms underlying sleep regulation, however, remain largely unclear. In the nematode Caenorhabditis elegans, developmentally timed sleep (DTS) and stress-induced sleep (SIS) are 2 types of quiescent behaviors that fulfill the definition of sleep and share conserved sleep-regulating molecules with mammals. To identify novel sleep-regulating molecules, we conducted an unbiased forward genetic screen based on DTS phenotypes. We isolated 2 mutants, rem8 and rem10, that exhibited significantly disrupted DTS and SIS. The causal gene of the abnormal sleep phenotypes in both mutants was mapped to dgk-1, which encodes diacylglycerol kinase. Perhaps due to the diminished SIS, dgk-1 mutant worms exhibited decreased survival following exposure to a noxious stimulus. Pan-neuronal and/or cholinergic expression of dgk-1 partly rescued the dgk-1 mutant defects in DTS, SIS, and post-stress survival. Moreover, we revealed that pkc-1/nPKC participates in sleep regulation and counteracts the effect of dgk-1; the reduced DTS, SIS, and post-stress survival rate were partly suppressed in the pkc-1; dgk-1 double mutant compared with the dgk-1 single mutant. Excessive sleep observed in the pkc-1 mutant was also suppressed in the pkc-1; dgk-1 double mutant, implying that dgk-1 has a complicated mode of action. Our findings indicate that neuronal DGK-1 is essential for normal sleep and that the counterbalance between DGK-1 and PKC-1 is crucial for regulating sleep and mitigating post-stress damage.

Stem cell-specific ecdysone signaling regulates the development and function of a Drosophila sleep homeostat

Complex behaviors arise from neural circuits that are assembled from diverse cell types. Sleep is a conserved and essential behavior, yet little is known regarding how the nervous system generates neuron types of the sleep-wake circuit. Here, we focus on the specification of Drosophila sleep-promoting neurons-long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex (CX). We use lineage analysis and genetic birth dating to identify two bilateral Type II neural stem cells that generate these dorsal fan-shaped body (dFB) neurons. We show that adult dFB neurons express Ecdysone-induced protein E93, and loss of Ecdysone signaling or E93 in Type II NSCs results in the misspecification of the adult dFB neurons. Finally, we show that E93 knockdown in Type II NSCs affects adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.

Sleep: An Essential and Understudied Process in the Biology of Blood-Feeding Arthropods

Understanding the biology of blood-feeding arthropods is critical to managing them as vectors of etiological agents. Circadian rhythms act in the regulation of behavioral and physiological aspects such as blood feeding, immunity, and reproduction. However, the impact of sleep on these processes has been largely ignored in blood-feeding arthropods, but recent studies in mosquitoes show that sleep-like states directly impact host landing and blood feeding. Our focus in this review is on discussing the relationship between sleep and circadian rhythms in blood-feeding arthropods along with how unique aspects such as blood gluttony and dormancy can impact sleep-like states. We highlight that sleep-like states are likely to have profound impacts on vector-host interactions but will vary between lineages even though few direct studies have been conducted. A myriad of factors, such as artificial light, could directly impact the time and levels of sleep in blood-feeding arthropods and their roles as vectors. Lastly, we discuss underlying factors that make sleep studies in blood-feeding arthropods difficult and how these can be bypassed. As sleep is a critical factor in the fitness of animal systems, a lack of focus on sleep in blood-feeding arthropods represents a significant oversight in understanding their behavior and its role in pathogen transmission.

Sleep need, the key regulator of sleep homeostasis, is indicated and controlled by phosphorylation of threonine 221 in salt-inducible kinase 3

Sleep need drives sleep and plays a key role in homeostatic regulation of sleep. So far sleep need can only be inferred by animal behaviors and indicated by electroencephalography (EEG). Here we report that phosphorylation of threonine (T) 221 of the salt-inducible kinase 3 (SIK3) increased the catalytic activity and stability of SIK3. T221 phosphorylation in the mouse brain indicates sleep need: more sleep resulting in less phosphorylation and less sleep more phosphorylation during daily sleep/wake cycle and after sleep deprivation (SD). Sleep need was reduced in SIK3 loss of function (LOF) mutants and by T221 mutation to alanine (T221A). Rebound after SD was also decreased in SIK3 LOF and T221A mutant mice. By contrast, SIK1 and SIK2 do not satisfy criteria to be both an indicator and a controller of sleep need. Our results reveal SIK3-T221 phosphorylation as a chemical modification which indicates and controls sleep need.

The neuropeptide receptor npr-38 regulates avoidance and stress-induced sleep in Caenorhabditis elegans

Although essential and conserved, sleep is not without its challenges that must be overcome; most notably, it renders animals vulnerable to threats in the environment. Infection and injury increase sleep demand, which dampens sensory responsiveness to stimuli, including those responsible for the initial insult. Stress-induced sleep in Caenorhabditis elegans occurs in response to cellular damage following noxious exposures the animals attempted to avoid. Here, we describe a G-protein-coupled receptor (GPCR) encoded by npr-38, which is required for stress-related responses including avoidance, sleep, and arousal. Overexpression of npr-38 shortens the avoidance phase and causes animals to initiate movement quiescence and arouse early. npr-38 functions in the ADL sensory neurons, which express neuropeptides encoded by nlp-50, also required for movement quiescence. npr-38 regulates arousal by acting on the DVA and RIS interneurons. Our work demonstrates that this single GPCR regulates multiple aspects of the stress response by functioning in sensory and sleep interneurons.

A familial natural short sleep mutation promotes healthy aging and extends lifespan in Drosophila

Sleep loss typically imposes negative effects on animal health. However, humans with a rare genetic mutation in the dec2 gene (dec2P384R) present an exception; these individuals sleep less without the usual effects associated with sleep deprivation. Thus, it has been suggested that the dec2P384R mutation activates compensatory mechanisms that allows these individuals to thrive with less sleep. To test this directly, we used a Drosophila model to study the effects of the dec2P384R mutation on animal health. Expression of human dec2P384R in fly sleep neurons was sufficient to mimic the short sleep phenotype and, remarkably, dec2P384R mutants lived significantly longer with improved health despite sleeping less. The improved physiological effects were enabled, in part, by enhanced mitochondrial fitness and upregulation of multiple stress response pathways. Moreover, we provide evidence that upregulation of pro-health pathways also contributes to the short sleep phenotype, and this phenomenon may extend to other pro-longevity models.

Behavioral screening of sleep-promoting effects of human intestinal and food-associated bacteria on Drosophila melanogaster

Commensal microbes influence various aspects of vertebrate and invertebrate brain function. We previously reported that Lactiplantibacillus plantarum SBT2227 promotes sleep in the fruit fly, Drosophila melanogaster. However, how widely the sleep-promoting effects are conserved in gut bacterial species remains unknown. In this study, we orally administered human intestinal and food-associated bacterial species (39 in total) to flies and investigated their effects on sleep. Six species of bacteria were found to have significant sleep-promoting effects. Of these, we further investigated Bifidobacterium adolescentis, which had the greatest sleep-promoting effect, and found that the strength of the sleep effect varied among strains of the same bacterial species. The B. adolescentis strains BA2786 and BA003 showed strong and weak effects on sleep, respectively. Transcriptome characteristics compared between the heads of flies treated with BA2786 or BA003 revealed that the gene expression of the insulin-like receptor (InR) was increased in BA2786-fed flies. Furthermore, a heterozygous mutation in InR suppressed the sleep-promoting effect of BA2786. These results suggest that orally administered sleep-promoting bacteria (at least BA2786), may act on insulin signaling to modulate brain function for sleep.

Tfap2b acts in GABAergic neurons to control sleep in mice

Sleep is a universal state of behavioral quiescence in both vertebrates and invertebrates that is controlled by conserved genes. We previously found that AP2 transcription factors control sleep in C. elegans, Drosophila, and mice. Heterozygous deletion of Tfap2b, one of the mammalian AP2 paralogs, reduces sleep in mice. The cell types and mechanisms through which Tfap2b controls sleep in mammals are, however, not known. In mice, Tfap2b acts during early embryonic stages. In this study, we used RNA-seq to measure the gene expression changes in brains of Tfap2b^-/- embryos. Our results indicated that genes related to brain development and patterning were differentially regulated. As many sleep-promoting neurons are known to be GABAergic, we measured the expression of GAD1, GAD2 and Vgat genes in different brain areas of adult Tfap2b^+/- mice using qPCR. These experiments suggested that GABAergic genes are downregulated in the cortex, brainstem and cerebellum areas, but upregulated in the striatum. To investigate whether Tfap2b controls sleep through GABAergic neurons, we specifically deleted Tfap2b in GABAergic neurons. We recorded the EEG and EMG before and after a 6-h period of sleep deprivation and extracted the time spent in NREM and in REM sleep as well as delta and theta power to assess NREM and REM sleep, respectively. During baseline conditions, Vgat-tfap2b^-/- mice exhibited both shortened NREM and REM sleep time and reduced delta and theta power. Consistently, weaker delta and theta power were observed during rebound sleep in the Vgat-tfap2b^-/- mice after sleep deprivation. Taken together, the results indicate that Tfap2b in GABAergic neurons is required for normal sleep.