Sleep Physiology in Toddlers: Effects of Missing a Nap on Subsequent Night Sleep

Genetic contribution to sleep homeostasis in early adolescence

The sleep homeostatic process in adults is moderately stable over time and unique to an individual. Work in transgenic mice has suggested a role of genes in sleep homeostasis. The current study quantified the genetic contribution to sleep homeostasis in adolescence. We use slow wave energy (SWE) as a metric for sleep pressure dissipation during sleep. This measure reflects both sleep intensity and duration. High-density (58 derivations) sleep electroencephalogram (EEG) was recorded in 14 monozygotic and 12 dizygotic adolescent twin pairs (mean age = 13.2 years; standard deviation [SD] = 1.1; 20 females). SWE at the end of sleep was quantified as the cumulative delta power (1-4.6 Hz) over the night. We also examined the time constant of the decay and the level of slow wave activity (SWA) at the beginning of the sleep episode. Structural equation modelling was used to quantify the amount of variance in SWE and the dissipation of sleep pressure due to genes. We found that most (mean = 76% across EEG derivations) of the variance in SWE was due to genes. In contrast, genes had a small (mean = 33%) influence on the rate of dissipation of sleep pressure, and this measure was largely (mean = 67%) driven by environmental factors unique to each twin. Our results show that the amount of dissipated sleep pressure is largely under genetic control; however, the rate of sleep pressure dissipation is largely due to unique environmental factors. Our findings are in line with research in animals and suggest that the heritability of the rate of sleep pressure dissipation is limited.

Bedtime to the brain: how infants' sleep behaviours intertwine with non-rapid eye movement sleep electroencephalography features

Adequate sleep is critical for development and facilitates the maturation of the neurophysiological circuitries at the basis of cognitive and behavioural function. Observational research has associated early life sleep problems with worse later cognitive, psychosocial, and somatic health outcomes. Yet, the extent to which day-to-day sleep behaviours (e.g., duration, regularity) in early life relate to non-rapid eye movement (NREM) neurophysiology-acutely and the long-term-remains to be studied. We measured sleep behaviours in 32 healthy 6-month-olds assessed with actimetry and neurophysiology with high-density electroencephalography (EEG) to investigate the association between NREM sleep and habitual sleep behaviours. Our study revealed four findings: first, daytime sleep behaviours are related to EEG slow-wave activity (SWA). Second, night-time movement and awakenings from sleep are connected with spindle density. Third, habitual sleep timing is linked to neurophysiological connectivity quantified as delta coherence. And lastly, delta coherence at 6 months predicts night-time sleep duration at 12 months. These novel findings widen our understanding that infants' sleep behaviours are closely intertwined with three particular levels of neurophysiology: sleep pressure (determined by SWA), the maturation of the thalamocortical system (spindles), and the maturation of cortical connectivity (coherence). The crucial next step is to extend this concept to clinical groups to objectively characterise infants' sleep behaviours 'at risk' that foster later neurodevelopmental problems.

Lack of association between behavioral development and simplified topographical markers of the sleep EEG in infancy

The sleep EEG mirrors neuronal connectivity, especially during development when the brain undergoes substantial rewiring. As children grow, the slow-wave activity (SWA; 0.75-4.25 Hz) spatial distribution in their sleep EEG changes along a posterior-to-anterior gradient. Topographical SWA markers have been linked to critical neurobehavioral functions, such as motor skills, in school-aged children. However, the relationship between topographical markers in infancy and later behavioral outcomes is still unclear. This study aims to explore reliable indicators of neurodevelopment in infants by analyzing their sleep EEG patterns. Thirty-one 6-month-old infants (15 female) underwent high-density EEG recordings during nighttime sleep. We defined markers based on the topographical distribution of SWA and theta activity, including central/occipital and frontal/occipital ratios and an index derived from local EEG power variability. Linear models were applied to test whether markers relate to concurrent, later, or retrospective behavioral scores, assessed by the parent-reported Ages & Stages Questionnaire at ages 3, 6, 12, and 24 months. Results indicate that the topographical markers of the sleep EEG power in infants were not significantly linked to behavioral development at any age. Further research, such as longitudinal sleep EEG in newborns, is needed to better understand the relationship between these markers and behavioral development and assess their predictive value for individual differences.

Sleep Architecture and Sleep EEG Alterations are Associated with Impaired Cognition Under Sleep Restriction

Purpose

Many studies have investigated the cognitive, emotional, and other impairments caused by sleep restriction. However, few studies have explored the relationship between cognitive performance and changes in sleep structure and electroencephalography (EEG) during sleep. The present study aimed to examine whether changes in sleep structure and EEG can account for cognitive impairment caused by sleep restriction.

Patients and Methods

Sixteen young adults spent five consecutive nights (adaptation 9h, baseline 8h, 1st restriction 6h, 2nd restriction 6h, and recovery 10h) in a sleep laboratory, with polysomnography recordings taken during sleep. Throughout waking periods in each condition, participants completed the psychomotor vigilance test (PVT), which measures vigilant attention, and the Go/No-Go task, which measures inhibition control.

Results

The results showed that sleep restriction significantly decreased the proportion of N1 and N2 sleep, increased the proportion of N3 sleep, and reduced the time spent awake after sleep onset (WASO) and sleep onset latency. Poorer performance on the PVT and Go/No Go task was associated with longer WASO, a larger proportion of N3 sleep, and a smaller proportion of N2 sleep. Additionally, the power spectral density of delta waves significantly increased after sleep restriction, and this increase predicted a decrease in vigilance and inhibition control the next day.

Conclusion

These findings suggest that sleep architecture and EEG signatures may partially explain cognitive impairment caused by sleep restriction.

Sleep from Infancy Through Adolescence

This article describes the changes in normal sleep regulation, structure, and organization and sleep-related changes in respiration from infancy to adolescence. The first 2 years of age are striking, with more time asleep than awake. With development, the electroencephalogram architecture has a marked reduction in rapid eye movement sleep and the acquisition of K-complexes, sleep spindles, and slow-wave sleep. During adolescence there is a reduction in slow-wave sleep and a delay in the circadian phase. Infants have a more collapsible upper airway and lower lung volumes than older children, which predisposes them to obstructive sleep apnea and sleep-related hypoxemia.

Circle(s) of Life: The Circadian Clock from Birth to Death

Most lifeforms on earth use endogenous, so-called circadian clocks to adapt to 24-h cycles in environmental demands driven by the planet’s rotation around its axis. Interactions with the environment change over the course of a lifetime, and so does regulation of the circadian clock system. In this review, we summarize how circadian clocks develop in humans and experimental rodents during embryonic development, how they mature after birth and what changes occur during puberty, adolescence and with increasing age. Special emphasis is laid on the circadian regulation of reproductive systems as major organizers of life segments and life span. We discuss differences in sexes and outline potential areas for future research. Finally, potential options for medical applications of lifespan chronobiology are discussed.

Contributions of memory and brain development to the bioregulation of naps and nap transitions in early childhood

The transition from multiple sleep bouts each day to a single overnight sleep bout (i.e., nap transition) is a universal process in human development. Naps are important during infancy and early childhood as they enhance learning through memory consolidation. However, a normal part of development is the transition out of naps. Understanding nap transitions is essential in order to maximize early learning and promote positive long-term cognitive outcomes. Here, we propose a novel hypothesis regarding the cognitive, physiological, and neural changes that accompany nap transitions. Specifically, we posit that maturation of the hippocampal-dependent memory network results in more efficient memory storage, which reduces the buildup of homeostatic sleep pressure across the cortex (as reflected by slow-wave activity), and eventually, contributes to nap transitions. This hypothesis synthesizes evidence of bioregulatory mechanisms underlying nap transitions and sheds new light on an important window of change in development. This framework can be used to evaluate multiple untested predictions from the field of sleep science and ultimately, yield science-based guidelines and policies regarding napping in childcare and early education settings.

Sleep in children with refractory epilepsy and epileptic encephalopathies: A systematic review of literature

Children with epilepsy have more sleep disorders compared to healthy children. The bidirectional interaction between epilepsy and sleep is not completely understood. However, disruption of sleep architecture during childhood may have consequences for cognitive development. As children with drug-refractory epilepsy often have intellectual disability, sleep disruption could be an important contributing factor in severity of their cognitive impairment. To better understand these interactions, sleep architecture in children with drug-refractory epilepsy and epileptic encephalopathies should be investigated. In this review, we conducted a systematic literature search on this topic. Articles that investigated sleep macro- and/or microstructure by means of electroencephalogram/polysomnography were included, as well as articles that used validated questionnaires. Sixteen articles were reviewed, eight of which used polysomnography. Only 2 articles examined sleep in children with epileptic encephalopathies. Consistent findings on measures of sleep architecture were a reduction in REM percentage and an increase in sleep fragmentation when comparing drug-refractory patients with non-refractory and healthy subjects. The findings on slow wave sleep were less clear. Studies with questionnaires unambiguously confirmed subjectively more sleep problems in children with drug-refractory epilepsy. This is the first review of literature in this patient population. More good quality sleep studies in children with drug-refractory epilepsy are warranted. The use of wearables in the home setting together with automatic sleep staging could provide more insights.

Does the maturation of early sleep patterns predict language ability at school entry? A Born in Bradford study

Children's vocabulary ability at school entry is highly variable and predictive of later language and literacy outcomes. Sleep is potentially useful in understanding and explaining that variability, with sleep patterns being predictive of global trajectories of language acquisition. Here, we looked to replicate and extend these findings. Data from 354 children (without English as an additional language) in the Born in Bradford study were analysed, describing the mean intercepts and linear trends in parent-reported day-time and night-time sleep duration over five time points between 6 and 36 months-of-age. The mean difference between night-time and day-time sleep was predictive of receptive vocabulary at age five, with more night-time sleep relative to day-time sleep predicting better language. An exploratory analysis suggested that socioeconomic status was predictive of vocabulary outcomes, with sleep patterns partially mediating this relationship. We suggest that the consolidation of sleep patterns acts as a driver of early language development.

Actigraphy in sleep research with infants and young children: Current practices and future benefits of standardized reporting

Actigraphy is a cost-efficient method to estimate sleep-wake patterns over long periods in natural settings. However, the lack of methodological standards in actigraphy research complicates the generalization of outcomes. A rapidly growing methodological diversity is visible in the field, which increasingly necessitates the detailed reporting of methodology. We address this problem and evaluate the current state of the art and recent methodological developments in actigraphy reporting with a special focus on infants and young children. Through a systematic literature search on PubMed (keywords: sleep, actigraphy, child *, preschool, children, infant), we identified 126 recent articles (published since 2012), which were classified and evaluated for reporting of actigraphy. Results show that all studies report on the number of days/nights the actigraph was worn. Reporting was good with respect to device model, placement and sleep diary, whereas reporting was worse for epoch length, algorithm, artefact identification, data loss and definition of variables. In the studies with infants only (n = 58), the majority of articles (62.1%) reported a recording of actigraphy that was continuous across 24 hr. Of these, 23 articles (63.9%) analysed the continuous 24-hr data and merely a fifth used actigraphy to quantify daytime sleep. In comparison with an evaluation in 2012, we observed small improvements in reporting of actigraphy methodology. We propose stricter adherence to standards in reporting methodology in order to streamline actigraphy research with infants and young children, to improve comparability and to facilitate big data ventures in the sleep community.

Homeostatic Response to Sleep Restriction in Adolescents

The high prevalence of chronic sleep restriction in adolescents underscores the importance of understanding how adolescent sleep is regulated under such conditions. One component of sleep regulation is a homeostatic process: if sleep is restricted, then sleep intensity increases. Our knowledge of this process is primarily informed by total sleep deprivation studies and has been incorporated in mathematical models of human sleep regulation. Several animal studies, however, suggest that adaptation occurs in chronic sleep restriction conditions, showing an attenuated or even decreased homeostatic response. We investigated the homeostatic response of adolescents to different sleep opportunities. Thirty-four participants were allocated to one of three groups with 5, 7.5 or 10 h of sleep opportunity per night for 5 nights. Each group underwent a protocol of 9 nights designed to mimic a school week between 2 weekends: 2 baseline nights (10 h sleep opportunity), 5 condition nights (5, 7.5 or 10 h), and two recovery nights (10 h). Measures of sleep homeostasis (slow-wave activity and slow-wave energy) were calculated from frontal and central EEG derivations and compared to predictions derived from simulations of the homeostatic process of the two-process model of sleep regulation. Only minor differences were found between empirical data and model predictions, indicating that sleep homeostasis is preserved under chronic sleep restriction in adolescents. These findings improve our understanding of effects of repetitive short sleep in adolescents.

Sleep architecture and homeostasis in children with epilepsy: a neurodevelopmental perspective

Although the influence of sleep on epilepsy has long been recognized, this relationship has yet to be fully exploited to benefit patients. The past decade has seen significant advances in understanding paediatric sleep, providing a framework by which to properly evaluate the sleep of children with epilepsy, which itself has been subject to increasing scrutiny. The role of sleep in learning and the potential for interictal discharges to disrupt sleep-related memory consolidation provide a novel perspective for understanding the association of childhood epilepsy with a high rate of intellectual disability. In this review, I outline the evolution of sleep duration, architecture, and homeostasis across childhood, relating this to the development of cognitive functions. I describe how these may be disrupted or preserved in children with epilepsy; in particular, collating data from polysomnography. Finally, I explore how sleep may, in the future, be modulated to improve cognitive outcome in these patients. WHAT THIS PAPER ADDS: Children with epilepsy have less rapid eye movement sleep than controls, but this improves with seizure cessation. Deep or slow-wave sleep is highly conserved in children with epilepsy. Sleep homeostasis may be disrupted either at a local or global level by the presence of interictal epileptiform discharges.

Sleep in children with Smith–Magenis syndrome: a case–control actigraphy study

Study Objectives

The objectives of the study were (1) to compare both actigraphy and questionnaire-assessed sleep quality and timing in children with Smith–Magenis syndrome (SMS) to a chronologically age-matched typically developing (TD) group and (2) to explore associations between age, nocturnal and diurnal sleep quality, and daytime behavior.

Methods

Seven nights of actigraphy data were collected from 20 children with SMS (mean age 8.70; SD 2.70) and 20 TD children. Daily parent/teacher ratings of behavior and sleepiness were obtained. Mixed linear modeling was used to explore associations between total sleep time and daytime naps and behavior.

Results

Sleep in children with SMS was characterized by shorter total sleep time (TST), extended night waking, shorter sleep onset, more daytime naps, and earlier morning waking compared to the TD group. Considerable inter-daily and inter-individual variability in sleep quality was found in the SMS group, so caution in generalizing results is required. An expected inverse association between age and TST was found in the TD group, but no significant association was found for the SMS group. No between-group differences in sleep hygiene practices were identified. A bidirectional negative association between TST and nap duration was found for the SMS group. In the SMS group, increased afternoon sleepiness was associated with increased irritability (p = .007) and overactivity (p = .005).

Conclusion

These findings evidence poor sleep quality in SMS and the need to implement evidence-based interventions in this population.

Sensitivity of the circadian system to evening bright light in preschool-age children

Although the light-induced melatonin suppression response is well characterized in adults, studies examining the dynamics of this effect in children are scarce. The purpose of this study was to quantify the magnitude of evening light-induced melatonin suppression in preschool-age children. Healthy children (n = 10; 7 females; 4.3 ± 1.1 years) participated in a 7-day protocol. On days 1-5, children followed a strict sleep schedule. On day 6, children entered a dim light environment (<15 lux) for 1-h before providing salivary samples every 20- to 30-min from the afternoon until 50-min after scheduled bedtime. On day 7, subjects remained in dim light conditions until 1-h before bedtime, at which time they were exposed to a bright light stimulus (~1000 lux) for 1-h and then re-entered dim light conditions. Saliva samples were obtained before, during, and after bright light exposure and were time anchored to samples taken the previous evening. We found robust melatonin suppression (87.6 ± 10.0%) in response to the bright light stimulus. Melatonin levels remained attenuated for 50-min after termination of the light stimulus (P < 0.008). Furthermore, melatonin levels did not return to 50% of those observed in the dim light condition 50-min after the light exposure for 7/10 children. Our findings demonstrate a robust light-induced melatonin suppression response in preschool-age children. These findings have implications for understanding the role of evening light exposure in the development of evening settling difficulties and may serve as experimental evidence to support recommendations regarding light exposure and sleep hygiene practices in early childhood.

Developmental Changes in Ultradian Sleep Cycles across Early Childhood

Nocturnal human sleep is composed of cycles between rapid eye movement (REM) sleep and non-REM (NREM) sleep. In adults, the structure of ultradian cycles between NREM and REM sleep is well characterized; however, less is known about the developmental trajectories of ultradian sleep cycles across early childhood. Cross-sectional studies indicate that the rapid ultradian cycling of active-quiet sleep in infancy shifts to a more adult-like pattern of NREM-REM sleep cycling by the school-age years, yet longitudinal studies elucidating the details of this transition are scarce. To address this gap, we examined ultradian cycling during nocturnal sleep following 13 h of prior wakefulness in 8 healthy children at 3 longitudinal points: 2Y (2.5-3.0 years of age), 3Y (3.5-4.0 years of age), and 5Y (5.5-6.0 years of age). We found that the length of ultradian cycles increased with age as a result of increased NREM sleep episode duration. In addition, we observed a significant decrease in the number of NREM sleep episodes as well as a nonsignificant trend for a decrease in the number of cycles with increasing age. Together, these findings suggest a concurrent change in which cycle duration increases and the number of cycles decreases across development. We also found that, consistent with data from adolescents and adults, the duration of NREM sleep episodes decreased with time since lights-off whereas the duration of REM sleep episodes increased over this time period. These results indicate the presence of circadian modulation of nocturnal sleep in preschool children. In addition to characterizing changes in ultradian cycling in healthy children ages 2 to 5 years, this work describes a developmental model that may provide insights into the emergence of normal adult REM sleep regulatory circuitry as well as potential trajectories of dysregulated ultradian cycles such as those associated with affective disorders.