Stability, precision, and near-24-hour period of the human circadian pacemaker

Impact of seasons and temperatures on the sleep-wake cycle in a French elderly rural population

Sleep is known to be affected by season changes in a temperate climate. Temperature changes are known to affect sleep directly, but in this context, season-related changes in the circadian rhythm may play an important role as well. The objectives of this study were to verify the effect of season and temperature on sleep parameters of elderly French subjects and to focus on the sleep-wake cycle. Sleep parameters and sleep-wake cycle parameters were analyzed through actigraphy while seasonality and temperature were acquired from recording dates and weather records from online scientific archives. ANOVAs were carried out to investigate the effect of seasons on actigraphic parameters and to calculate regression models for temperature. A sample of 157 subjects (49.7 % women) participated in the study with a mean age of 76.9 ± 4.5 years. Inter-daily stability of the sleep-wake cycle was higher in autumn and winter with a higher mean activity during those months. Time in bed was significantly longer in autumn and winter while there was a similar trend for total sleep time. Those variables changing with the seasons were negatively linked to ambient temperature. Not only sleep but also the sleep-wake cycle is impacted by seasonal changes in elderly French subjects. Seasons should be taken into consideration when planning sleep-wake cycle recordings in temperate climates, in particular for longitudinal protocols. Clinical interventions should take season-related sleep-wake cycle problems into account, particularly in the elderly, who suffer more often from sleep disorders than other age groups. Since elderly also suffer more from climate change effects, this study further adds to the demand for clinical monitoring and housing adaptations for the elderly in the future.

Daytime aversive stimuli do not phase shift behavioral rhythms under light-dark cycles in a strictly diurnal rodent

Recent studies have shown that cyclic aversive stimuli (time-specific footshocks) act as a nonphotic zeitgeber, shifting circadian behaviors to the daytime in nocturnal rodents through entrainment. It has remained untested whether diurnal species exhibit similar plasticity in behavioral timing. This study investigated whether antelope ground squirrels (Ammospermophilus leucurus, AGS), naturally diurnal rodents, shift activity timing in response to cyclic aversive stimuli delivered at specific phases of the light-dark (LD) cycle. We conducted two experiments with 20 AGS housed in custom cages featuring a safe nesting area and a separate foraging area rendered potentially aversive by unsignaled time-specific footshocks. In Experiment 1, animals were subjected to a 12:12 LD cycle. One group was exposed to a foraging area that produced footshocks during the light phase, and a control group with footshocks during the dark phase. In Experiment 2, under a 16:8 LD cycle, animals were divided into three groups, with footshock exposure either during the first or second half of the light phase or during the dark phase. Following treatments, animals were released into constant darkness (DD) to assess free-running rhythms. Contrary to findings in nocturnal rodents, AGS did not exhibit consistent complementary shifts to nocturnal activity as an avoidance of footshocks received during daytime. Most animals maintained diurnal activity, showing minor, and inconsistent phase adjustments. In Experiment 2, animals exposed to footshocks during part of the light phase also failed to reliably shift activity to the "safe" portion of the light phase. These findings show AGS do not substantially shift activity patterns in response to cyclic aversive stimuli and that a 24-h cyclic fear stimulus fails to override the LD cycle as a zeitgeber. This suggests a lack of plasticity in circadian behavior and highlights the importance of species-specific differences in response to potential nonphotic zeitgebers.

An exploratory pilot study on social rhythm regularity, and its associations with sleep, circadian, affective, and alcohol use outcomes in late adolescents

The current exploratory pilot study examined whether social rhythm regularity, as measured by a social rhythm metric, was associated with: (1) the regularity of circadian rhythms and/or sleep regularity metrics; and (2) sleep quality, affective function and alcohol use. Late adolescents (18-22 years old) who drink alcohol (n = 36; 61.1% female, Mage = 21.26 years) completed a 14-day ecological momentary assessment protocol, wore a wrist actigraph for 14 days, and completed two overnight visits (Thursday and Sunday) to assess dim light melatonin onset. Sleep regularity metrics included standard deviation, composite phase deviation, social jet lag and inter-daily stability. We used dim light melatonin onset data to calculate the stability of the circadian phase (Sunday minus Thursday). Participants completed surveys and ecological momentary assessments that assessed global and daily sleep quality, affective function, and alcohol use. Correlational analysis and robust regression modelling were used. More regular social rhythms were associated with higher regularities of mid-sleep timing based on standard deviations, but were not associated with other sleep regularity metrics or stability of the circadian phase. More regular social rhythms were associated with better sleep quality, but were not associated with affective function or alcohol use. Social rhythm regularity is a unique construct compared with existing sleep quality metrics. In contrast with the social zeitgeber hypothesis, social rhythm regularity was not associated with circadian rhythm regularity measured by dim light melatonin onset. However, social rhythm regularity may be an under-recognized contributor to better sleep quality.

The British Sleep Society position statement on Daylight Saving Time in the UK

There is an ongoing debate in the United Kingdom and in other countries about whether twice-yearly changes into and out of Daylight Saving Time should be abolished. Opinions are divided about whether any abolition of Daylight Saving Time should result in permanent Standard Time, or year-long Daylight Saving Time. The British Sleep Society concludes from the available scientific evidence that circadian and sleep health are affected negatively by enforced changes of clock time (especially in a forward direction) and positively by the availability of natural daylight during the morning. Thus, our recommendation is that the United Kingdom should abolish the twice-yearly clock change and reinstate Standard Time throughout the year.

Temperature as a circadian timing cue in the visually impaired

The daily rise and fall in ambient temperature caused by Earth's 24-hour rotation may help regulate circadian rhythms in visually impaired individuals. In all mammals, circadian rhythms, the daily cycles of physiology and behavior, are time controlled by the suprachiasmatic nucleus (SCN), the brain's central clock. The SCN typically synchronizes circadian rhythms with the light/dark cycle through photoentrainment, a process in which specialized retinal cells capture ambient light and transmit this information to the SCN, allowing it to set its phase. Without light input, the rodent SCN's light-driven circuits can become desynchronized, potentially allowing alternative entrainment signals, such as ambient temperature, to influence central timing. Here, we consider whether a similar mechanism could benefit visually impaired humans who, due to retinal damage, have reduced or absent photic input to the central clock. Visually impaired individuals often experience circadian misalignment, whereby internal rhythms drift out of synchrony with the light-dark cycle, and we suggest that temperature information may mitigate some of this drift. Temperature entrainment could operate through heat shock pathways from the skin, via thermoregulatory brain regions with reciprocal connections to the SCN, or by shifting core body temperature through warm or cold baths, which can alter the phase of clocks in peripheral organs and potentially feedback to adjust central time. Given that temperature is a weaker cue than light, it remains unknown if, and to what extent, it may significantly impact central timing. However, if effective, temperature entrainment in the visually impaired could potentially improve circadian disorders, poor sleep, and adverse health outcomes associated with circadian dysfunction including depression, cognitive decline, and metabolic disorders, which are more prevalent in this population. Research is needed to confirm the long-term effectiveness of temperature as an entrainment cue in the visually impaired population, which may have broader implications for circadian timekeeping in mammals and the role of temperature in the absence of light.

Hourly level analysis of the effects of temperature extremes on emergency ambulance calls

Methods

We obtained hourly data on EACs, weather conditions, and air pollutants from Nanjing, a megacity in eastern China, during 2018-21. We first extracted data from the cold and warm seasons to quantify the hourly impact of extreme cold and heat on EACs and potential modifying factors using a distributed lag nonlinear model. Then, we used a random-effects meta-analysis model to pool the estimates of extreme temperatures on EACs from this Nanjing study and from studies identified in the published literature.

Results

The results showed that hourly exposure to extreme cold and heat increased the risk of EACs for all non-accidental causes, with relative risks (RRs) of 1.175 (95% confidence interval (CI) = 1.135, 1.216) at lag seven to 22 hours and 1.096 (95% CI = 1.048, 1.146) at lag zero to 10 hours, respectively. Stronger extreme heat-EAC associations were found between 16:00-24:00. The meta-analysis, which additionally included four prior studies, confirmed a significant association between hourly exposure to temperature extremes and EAC risk (RR = 1.155 for extreme cold and RR = 1.172 for extreme heat).

Conclusions

Our findings indicate that transient exposure to temperature extremes can increase the demand for EACs within a few hours, which may have implications for improving ambulance service efficiency and developing an ambulance early warning system under extreme weather conditions.

Intranasal and inhaled delivery systems for targeting circadian dysfunction in neurodegenerative disorders, perspective and future outlook

Synchronisation of the suprachiasmatic nucleus (SCN) driven endogenous clock, located within the central nervous system (CNS), and exogenous time cues, is essential for maintaining circadian rhythmicity, homeostasis and overall wellbeing. Disordered circadian rhythms have been associated with various conditions, inclusive of neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. Traditional pharmacological approaches to circadian dysfunction in neurodegenerative disorders have primarily focused on oral drug delivery. Oral medications often face challenges in achieving the necessary systemic circulation to effectively bypass the blood brain barrier (BBB) and reach the CNS, primarily due to low or variable bioavailability. Advancements in non-invasive delivery methods, such as orally inhaled and intranasal formulations, present promising alternatives for targeting the CNS. Orally inhaled and intranasal drug delivery allows for medications to rapidly achieve high systemic circulation through increased bioavailability and fast onset of action. Additionally, intranasal delivery allows for therapies to bypass the BBB through the olfactory or trigeminal nerve pathways to directly enter the CNS. This review assesses the potential for orally inhaled and intranasal therapies to treat circadian disorders in neurodegenerative conditions. In addition, this review will explore melatonin as an example of enhancing therapeutic outcomes by adopting inhaled or intranasal drug delivery formulations to improve drug absorption and target circadian disorder more effectively.

Group bright light therapy compared to treatment as usual for delayed sleep-wake phase disorder among patients in psychiatric care (the SIP trials): a protocol for a pragmatic, randomised controlled trial

INTRODUCTION

Circadian rhythm sleep-wake disturbances appear to be prevalent in psychiatric populations and may maintain and exacerbate psychiatric symptoms. Bright light therapy (BLT) is, in addition to exogenous melatonin, the treatment of choice for circadian rhythm disorders like delayed sleep-wake phase disorder (DSWPD) and has yielded promising results in patients with comorbid psychiatric illness. However, such patients are rarely offered this treatment in outpatient clinics. The aim of this randomised controlled trial is to investigate whether group BLT for psychiatric outpatients is superior to treatment as usual (TAU).

METHODS AND ANALYSIS

60 patients with moderate-to-severe psychiatric illness who meet the criteria for DSWPD will be recruited from an outpatient psychiatric clinic in Norway. They will be randomised (1:1) to a group-based Sleep School Wake Up! For Circadian (SSWU-C) programme conjointly with TAU or to TAU while on a wait list for SSWU-C. The SSWU-C will be delivered over four biweekly sessions, each lasting 120 min; hence treatment will last 6 weeks. Assessments will be collected at baseline (T1) and after the intervention (T2). The primary outcome will be changes in sleep timing using measures such as sleep diaries, actigraphy and dim light melatonin onset (DLMO) at 6 weeks postintervention. Secondary outcomes include changes in other sleep metrics, symptoms of depression, anxiety, fatigue, problems with work and social adjustment and well-being. Mixed models will be used for data analyses.

ETHICS AND DISSEMINATION

Ethical approval was granted in 2020 by the Regional Ethics Committee in Western Norway (REK 2020/66304). Findings will be published in peer-reviewed journals and be presented at research conferences and in relevant media. The results may document the need for more specific sleep-directed treatments in psychiatric clinics as a way of treating not only circadian rhythm sleep-wake disorders but also as a treatment to alleviate psychiatric symptoms.

TRIAL REGISTRATION NUMBER

NCT05177055.

Circadian Rhythm Dysregulation in Inflammatory Bowel Disease: Mechanisms and Chronotherapeutic Approaches

Inflammatory bowel disease (IBD), comprising ulcerative colitis (UC) and Crohn's disease (CD), is characterized by chronic intestinal inflammation. Recent research has highlighted the significant interplay between IBD pathogenesis and circadian rhythms. This review synthesizes current evidence regarding circadian regulation in IBD, covering three main areas: (1) circadian rhythms in intestinal physiology, (2) circadian disruption patterns in IBD patients, and (3) the role of clock genes in IBD pathogenesis. We discuss how these findings may inform novel chronotherapeutic approaches for IBD treatment. Future research directions that could facilitate translation of chronobiological insights into clinical applications are also explored.

Daytime eating during simulated night work mitigates changes in cardiovascular risk factors: secondary analyses of a randomized controlled trial

Effective countermeasures against the adverse cardiovascular effects of circadian misalignment, such as effects experienced due to night work or jet lag, remain to be established in humans. Here, we aim to test whether eating only during daytime can mitigate such adverse effects vs. eating during the night and day (typical for night shift workers) under simulated night work (secondary analysis of NCT02291952). This single-blind, parallel-arm trial randomized 20 healthy participants (non-shift workers) to simulated night work with meals consumed during night and day (Nighttime Meal Control Group) or only during daytime (Daytime Meal Intervention Group). The primary outcomes were pNN50 (percentage consecutive heartbeat intervals >50 ms), RMSSD (root mean square of successive heartbeat differences), and LF/HF (low/high cardiac frequency). The secondary outcome was blood concentrations of prothrombotic factor plasminogen activator inhibitor-1 (PAI-1). These measures were assessed under Constant Routine conditions, before (baseline) and after (postmisalignment) simulated night work. The meal timing intervention significantly modified the impact of simulated night work on cardiac vagal modulation and PAI-1 (pFDR = 0.001). In the Control Group, the postmisalignment Constant Routine showed a decrease in pNN50 by 25.7% (pFDR = 0.008) and RMMSD by 14.3% (pFDR = 0.02), and an increase in LF/HF by 5.5% (pFDR = 0.04) and PAI-1 by 23.9% (pFDR = 0.04), vs. the baseline Constant Routine. In the Intervention Group, there were no significant changes in these outcomes. For exploratory outcomes, the intervention significantly modified the impact of simulated night work on blood pressure (P < 0.05), with no significant change in the Control Group, and a significant reduction by 6-8% (P < 0.01) in the Intervention Group; without significant effects for heart rate or cortisol. These findings indicate that daytime eating, despite mistimed sleep, may mitigate changes in cardiovascular risk factors and offer translational evidence for developing a behavioral strategy to help minimize the adverse changes in cardiovascular risk factors in individuals exposed to circadian misalignment, such as shift workers.

Chronotherapeutic Treatments for Psychiatric Disorders: A Narrative Review of Recent Literature

PURPOSE OF THE REVIEW

This narrative review examines the newest findings from clinical trials of chronotherapeutics for psychiatric disorders. We reviewed the potential effects of the most commonly-studied chronotherapeutics such as bright light therapy, dark therapy, melatonin, and chronotherapy on the psychiatric disorders of depression, bipolar disorder, and anxiety disorders.

RECENT FINDINGS

The preponderance of recent clinical trials in chronotherapeutics has focused on bright light therapy in depression. However, there is an emerging body of preliminary studies testing chronotherapeutics in other psychiatric disorders, including bipolar disorder and post-traumatic stress disorder. Chronotherapeutics hold potential to improve sleep in adults with psychiatric conditions as well as psychiatric symptoms. Although the most recent literature demonstrates the promise of these interventions, the current body of work is limited by small sample sizes and relatively few studies outside of depression. Larger-scale trials are needed to refine treatment protocols, develop personalized treatment approaches, and inform dissemination. Studies in psychiatric conditions besides depression are particularly needed.

Circadian clock communication during homeostasis and ageing

Maintaining homeostasis is essential for continued health, and the progressive decay of homeostatic processes is a hallmark of ageing. Daily environmental rhythms threaten homeostasis, and circadian clocks have evolved to execute physiological processes in a manner that anticipates, and thus mitigates, their effects on the organism. Clocks are active in almost all cell types; their rhythmicity and functional output are determined by a combination of tissue-intrinsic and systemic inputs. Numerous inputs for a specific tissue are produced by the activity of circadian clocks of other tissues or cell types, generating a form of crosstalk known as clock communication. In mammals, the central clock in the hypothalamus integrates signals from external light-dark cycles to align peripheral clocks elsewhere in the body. This regulation is complemented by a tissue-specific milieu of external, systemic and niche inputs that modulate and cooperate with the cellular circadian clock machinery of a tissue to tailor its functional output. These mechanisms of clock communication decay during ageing, and growing evidence suggests that this decline might drive ageing-related morbidities. Dietary, behavioural and pharmacological interventions may offer the possibility to overcome these changes and in turn improve healthspan.

Clinical physiology of circadian rhythms: A systematic and hierarchized content analysis of circadian questionnaires

Circadian rhythms are near-24 h patterns of physiology and behavior associated with several physical and mental health outcomes. Self-report questionnaires are routinely used and practical tools to assess circadian rhythms. However, the extent to which these questionnaires capture the relevant parameters and can be used interchangeably is unknown. We investigated different types of circadian manifestations using 14 circadian self-report questionnaires for adults. A systematic and hierarchical content analysis was combined with a visualization method. Jaccard indices were calculated to quantify the degree to which the questionnaires overlapped. Content analysis revealed 40 distinct manifestations, which we classified into five dimensions ("circadian phase," "circadian amplitude and stability," "nycthemeral timing," "nycthemeral regularity," and "circadian complaints"). The average Jaccard index was 0.150, indicating very weak content overlap. None of the 14 questionnaires explored all five dimensions. The Composite Scale of Morningness and the Morningness-Eveningness Questionnaire exhibited greater, but still limited, overlap with the other questionnaires (Jaccard indices of 0.255 and 0.251, respectively), and are the best instruments for assessing the circadian phase. Nycthemeral timing, which must be analyzed to measure the circadian misalignment in clinical and research settings, is only explored by the Munich Chronotype Questionnaire, but that instrument does not evaluate circadian amplitude and stability and only partially assesses nycthemeral regularity. Based on our preliminary analysis, we make recommendations regarding the circumstances in which some circadian questionnaires could prove more useful than the others. The results might also aid the definition and investigation of circadian health at the crossroads of physiology and behavior.

Impact of long-haul airline travel on athletic performance and recovery: A critical review of the literature

Participation in many important sport events (e.g., World championships, Olympics) requires athletes to fly >4 h and to cross several time zones. This transmeridian travel results in a transient desynchronization of the body's circadian rhythms due to a disconnect between the timing of the endogenous circadian oscillator and the external stimuli, manifested as 'jet lag'. While recent reviews highlight the importance of managing jet lag, the time required for resynchronization of the internal clock and dissipation of jet lag symptoms has not yet been summarized. Further, although the literature reports that rapid transmeridian travel is detrimental for athletes' performance, empirical evidence from studies involving athletes is equivocal. Herein, we summarize the evidence that the variability in responses to transmeridian travel can be attributed to differences in (i) travel (real vs. simulated, westbound vs. eastbound, time zones crossed, during normal waking hours vs. normal sleep time), (ii) testing (assessment of performance vs. factors related to performance), and (iii) timing of the testing (destination time vs. 'body time'), and we offer the possibility that differences in (iv) teams, (v) traits, and (vi) tournaments may also be implicated. We focus on (i) aerobic power/endurance, (ii) anaerobic power and capacity, (iii) strength, and (iv) mood state, sleep quantity and quality, and jet lag symptoms in this literature review, which is limited to athletes or physically active participants, travelling west or east crossing four or more time zones.

Clinical Chronobiology: Circadian Rhythms in Health and Disease

Circadian rhythms (CRs) are entrainable endogenous rhythms that respond to external stimuli and regulate physiological functions. The suprachiasmatic nucleus (SCN) in the hypothalamus is the mammalian master clock that synchronizes all other tissue-specific peripheral clocks, primarily through gamma-aminobutyric acid (GABA) and vasoactive intestinal polypeptide (VIP). The SCN follows Earth's 24-hour cycle by light entrainment through the retinohypothalamic tract. At the cellular level, the core clock genes CLOCK, BMAL1, PER1-PER3, CRY1, and CRY2 regulate CRs in a negative feedback loop. The circadian disruption of the sleep-wake cycle manifests in at least six distinct clinical conditions. These are the circadian rhythm sleep-wake disorders (CRSWDs). Their diagnosis is made by history, sleep diaries, and actigraphy. Treatment involves a combination of timed light exposure, melatonin/melatonin agonists, and behavioral interventions. In addition, CR disturbances and subsequent misalignment can increase the risk of a variety of illnesses. These include infertility and menstrual irregularities as well as diabetes, obesity, fatty liver disease, and other metabolic syndromes. In addition, a disruption in the gut microbiome creates a proinflammatory environment. CR disturbances increase the risk for mood disorders, hence the utility of light-based therapies in depression. People with neurodegenerative disorders demonstrate significant disturbances in their CRs, and in their sleep-wake cycles. Circadian realignment therapies can also help decrease the symptomatic burden of these disorders. Certain epilepsy syndromes, such as juvenile myoclonic epilepsy (JME), have a circadian pattern of seizures. Circadian disturbances in epilepsy can be both the consequence and cause for breakthrough seizures. The immune system has its own CR. Disturbances in these due to shift work, for instance, can increase the risk of infections. CR disturbances can also increase the risk of cancer by impacting DNA repair, apoptosis, immune surveillance, and cell cycle regulation. Moreover, the timing of chemotherapeutic agents has been shown to increase their therapeutic impact in certain cancers.

Do not sleep on vitamin D: vitamin D is associated with sleep variability in apparently healthy adults

Vitamin D is associated with sleep quality and duration, but it is unclear whether vitamin D status influences sleep variability. Therefore, we sought to determine whether vitamin D status was associated with sleep variability in healthy adults. We assessed objective sleep, including timing and duration standard deviation (SD) using the Philips Actiwatch Spectrum and subjective sleep quality using the Pittsburgh Sleep Quality Index (PSQI) in 130 adults. We measured plasma 25(OH)D concentration to assess vitamin D. We used one-way ANOVAs and Kruskal-Wallis tests to compare sleep in participants characterized as vitamin D deficient (<20 ng/mL), insufficient (21-29 ng/mL), and sufficient (>30ng/mL). We used covariate-adjusted linear regression to assess associations between vitamin D status and sleep metrics. We compared differences in "low" and "high" sleep variability based on vitamin D status using the Chi-squared test. There was an effect of vitamin D status on sleep timing SD (Kruskal-Wallis, P = 0.021) and sleep duration SD (Kruskal-Wallis, P < 0.001). There was an inverse association between vitamin D status with sleep duration SD (after covariate adjustment R2 = 0.267, P < 0.001, deficient vs. sufficient P = 0.050, insufficient vs. sufficient P = 0.022). There was no effect of vitamin D status on objective sleep duration, efficiency, or PSQI scores (P > 0.05). We did not observe differences in "low" and "high" sleep timing SD based on vitamin D status (χ2 = 5.43, P = 0.066), but we did for sleep duration SD (χ2 = 22.4, P < 0.001). Our data indicate that individuals with poor vitamin D status exhibit greater objective sleep variability. Clinical Trial Registry: https://www.clinicaltrials.gov/; Unique identifiers NCT04334135, NCT04244604, and NCT04576338.NEW & NOTEWORTHY Our findings reveal that individuals with lower circulating vitamin D concentrations experience greater sleep variability compared with those with higher circulating concentrations. This supports the growing body of evidence suggesting an important link between vitamin D status and sleep health.

Temporal Tendencies: Exploring the Impact of Chronotype Timing on Youth Depression Risk

Developmental changes in youth sleep preferences (chronotype) and pubertal development are consequential for youth risk for depression. Previous research has identified individual differences in chronotype in risk for psychopathology. However, little is known regarding how the timing of chronotype may confer risk in youth. This study addressed this gap by examining associations between chronotypal timing and symptoms of depression in youth. Community youth (N = 155; M age =12.7) completed self-report measures of chronotype, pubertal status and depression every six months for a period of one year (three assessment points). Regression analyses showed that chronotypal timing predicted change in depressive symptoms across six months (b = -0.66, p = 0.019), but not across any other timeframe. Findings suggested that youth experiencing more of a morning preference compared to same-aged peers were at increased risk for later depression across six months. Chronotypal timing continued to predict changes in symptoms of depression controlling for gender (b = -0.63, p = 0.023) and pubertal timing (b = -0.72, p = 0.012). These findings suggest that chronotypal timing is prospectively related to changes in youth symptoms of depression. Results indicate that attention to the timing of normative changes in chronotype is warranted, in addition to mean-level differences.

Night shift work and breast cancer risk - 2023 update of epidemiologic evidence

Introduction

Night shift work is a complex and frequent occupational exposure, and breast cancer stands as the most prevalent cancer in women. The International Agency for Research on Cancer (IARC) has twice classified night shift work as a probable breast carcinogen, with the latest classification in June 2019. Since that time, new epidemiologic data has emerged.

Methods

We searched PubMed for original articles based on cohort and case-control studies of "breast cancer and night shift work" published after the IARC evaluation in June 2019.

Results

In total six cohorts and four case-control studies were included in our review. Overall, we observed some support for associations between persistent (long duration or high frequency) night shift work and an increase in breast cancer risk, though most studies were relatively small and statistically under-powered. Moreover, the recent studies do not contribute further evidence regarding the interaction with menopausal status, diurnal preference, hormonal subtypes of breast cancer or gene-environment aspects, which were issues that were left from the IARC evaluation.

Conclusions

The available new results somewhat consolidate the epidemiological evidence from IARC's 2019 evaluation, and do not provide further evidence regarding interaction of interest, e.g. menopausal status, etc. Therefore, long term follow-up of prospective cohorts or nested case-control studies, including precise exposure assessment and examinations of relevant interactions such as menopausal status, diurnal preference, hormonal subtypes of breast cancer and gene-environment aspects, are warranted. Meanwhile, protective measures for the night workers should be considered.

Chronodisruption that dampens output of the central clock abolishes rhythms in metabolome profiles and elevates acylcarnitine levels in the liver of female rats

AIM

Exposure to light at night and meal time misaligned with the light/dark (LD) cycle-typical features of daily life in modern 24/7 society-are associated with negative effects on health. To understand the mechanism, we developed a novel protocol of complex chronodisruption (CD) in which we exposed female rats to four weekly cycles consisting of 5-day intervals of constant light and 2-day intervals of food access restricted to the light phase of the 12:12 LD cycle.

METHODS

We examined the effects of CD on behavior, estrous cycle, sleep patterns, glucose homeostasis and profiles of clock- and metabolism-related gene expression (using RT qPCR) and liver metabolome and lipidome (using untargeted metabolomic and lipidomic profiling).

RESULTS

CD attenuated the rhythmic output of the central clock in the suprachiasmatic nucleus via Prok2 signaling, thereby disrupting locomotor activity, the estrous cycle, sleep patterns, and mutual phase relationship between the central and peripheral clocks. In the periphery, CD abolished Per1,2 expression rhythms in peripheral tissues (liver, pancreas, colon) and worsened glucose homeostasis. In the liver, it impaired the expression of NAD+, lipid, and cholesterol metabolism genes and abolished most of the high-amplitude rhythms of lipids and polar metabolites. Interestingly, CD abolished the circadian rhythm of Cpt1a expression and increased the levels of long-chain acylcarnitines (ACar 18:2, ACar 16:0), indicating enhanced fatty acid oxidation in mitochondria.

CONCLUSION

Our data show the widespread effects of CD on metabolism and point to ACars as biomarkers for CD due to misaligned sleep and feeding patterns.

Modeling of Jet Lag and Searching for an Optimal Light Treatment

The role of the hierarchical organization of the suprachiasmatic nucleus (SCN) in its functioning, jet lag, and the light treatment of jet lag remains poorly understood. Using the core-shell model, we mimic collective behavior of the core and shell populations of the SCN oscillators in transient states after rapid traveling east and west. The existence of a special region of slow dynamical states of the SCN oscillators can explain phenomena such as the east-west asymmetry of jet lag, instances when entrainment to an advance is via delay shifts, and the dynamics of jet lag recovery time. If jet lag brings the SCN state into this region, it will take a long time to leave it and restore synchronization among oscillators. We show that the population of oscillators in the core responds quickly to a rapid phase shift of the light-dark cycle, in contrast to the shell, which responds slowly. A slow recovery of the synchronization among the shell oscillators in transient states may strongly affect reentrainment in peripheral tissues and behavioral rhythms. We discuss the relationship between molecular, electrical, and behavioral rhythms. We also describe how light pulses affect the SCN and analyze the efficiency of the light treatment in facilitating the adaptation of the SCN to a new time zone. Light pulses of a moderate duration and intensity reduce the recovery time after traveling east, but not west. However, long duration and high intensity of light pulses are more detrimental than beneficial for speeding up reentrainment. The results of the core-shell model are compared with experimental data and other biologically motivated models of the SCN.

The relationship between the vestibular system and the circadian timing system: A review

This review attempts to analyze the relationship between the vestibular system and the circadian timing system. The activity of the biological clock allows an organism to optimally perform its tasks throughout the nychtemeron. To achieve this, the biological clock is subjected to exogenous factors that entrain it to a 24h period. While the most powerful synchronizer is the light-dark cycle produced by the Earth's rotation, research has led to the hypothesis of the vestibular system as a possible non-photic time cue used to entrain circadian rhythms. Demonstrated neuroanatomical pathways between vestibular nuclei and suprachiasmatic nuclei could transmit this message. Moreover, functional evidence in both humans and animals has shown that vestibular disruption or stimulation may lead to changes in circadian rhythms characteristics. Vestibular stimulations could be considered to act synergistically with other synchronizers, such as light, to ensure the entrainment of biological rhythms over the 24-h reference period.

A multi-center study of symptoms in patients with esophageal cancer postoperatively: A networking analysis

PURPOSE

This study aimed to explore symptom clusters and the inter-relationship of symptoms in esophageal cancer (EC) patients during the first week after surgery.

METHODS

A cross-sectional survey across multiple centers was carried out using the EORTCQLQ-OES18. Patients with esophageal cancer within a week post-surgery were recruited from the "Be Resilient to Cancer" project in Guangdong, Hunan, and Sichuan provinces between January and September 2024. Exploratory factor analysis with a priori algorithm was used to identify symptom clusters and network analysis was employed to recognize the relationship among core and bridge symptoms.

RESULTS

The sample consisted of 501 patients with esophageal cancer, who were predominantly male (83%), married (93%) and 57% were ≥60 years. Three symptom clusters were identified: "reflux-pain", "eating", and "dysphagia-dry mouth". Acid or bile coming up (support = 40.1%, confidence = 1, lift = 2.53), eating difficulties (support = 40.1%, confidence = 0.990, lift = 2.408) and dry mouth (support = 42.9%, confidence = 0.808, lift = 1.298) were marked as sentinel symptoms for these clusters, respectively. Acid indigestion or heartburn was identified as the core symptom (EI = 1.142 without covariates and EI = 1.153 with covariates), and dry mouth served as the bridge symptoms (EI = 0.63 and EI = 0.656).

CONCLUSIONS

Addressing acid or bile coming up, eating difficulties, dry mouth are imperative to help relief symptom burden at the cluster level. Furthermore, targeting acid indigestion and heartburn are crucial to break the chains among different symptom clusters.

Evaluation of circadian rhythm and prognostic variability pre-and post-CEA or CAS treatment in patients with carotid artery stenosis

Objective

Carotid artery stenosis, primarily caused by atherosclerosis, is a major risk factor for ischemic stroke. Carotid endarterectomy (CEA) and carotid artery stenting (CAS) are established interventions to reduce stroke risk and restore cerebral blood flow. However, the effect of these treatments on circadian rhythms, and their influence on stroke recovery, remains underexplored. This study aims to assess how disruptions in circadian rhythms-specifically sleep quality and blood pressure variability-impact recovery in patients undergoing CEA or CAS.

Methods

We conducted a prospective study involving 177 patients with carotid artery stenosis, all treated with either CEA or CAS. Patients were followed for 90 days post-treatment, with neurological outcomes evaluated using the NIHSS Stroke Scale (NIHSS). Circadian rhythm-related factors, including sleep quality (Pittsburgh Sleep Quality Index [PSQI]) and blood pressure variability (daytime systolic and nighttime diastolic BP), were assessed pre-and post-treatment. Stepwise regression was used to identify predictors of stroke recovery.

Results

In a cohort of 177 patients with symptomatic carotid atherosclerotic stenosis, stepwise regression identified post-treatment changes in PSQI, nighttime diastolic blood pressure, and the presence of coronary heart disease as significant independent predictors of poor neurological outcomes (p < 0.001). Both CEA and CAS significantly improved daytime systolic (p < 0.01) and nighttime diastolic blood pressure (p < 0.01). Patients with poorer prognosis had higher post-treatment PSQI scores (p < 0.001). Additionally, increased physical activity after treatment was linked to improved neurological recovery.

Conclusion

This study highlights the critical role of circadian rhythm regulation and cardiovascular health in stroke recovery following CEA or CAS. Stepwise regression analysis revealed that sleep quality, blood pressure stability, and coronary heart disease were key predictors of neurological outcomes, underscoring the importance of integrating circadian rhythm management into rehabilitation strategies. These results provide a robust scientific foundation for further investigation into the role of circadian rhythms in clinical practice.

Circadian rhythm disorders in the blind

Non-24-h sleep-wake disorder in blind patients without light perception is an orphan circadian rhythm sleep-wake disorder and is extremely rare in sighted people. Non-24-h sleep-wake disorder is characterized by insomnia and daytime sleepiness alternating with asymptomatic episodes. The frequency of symptomatic periods depends on the daily desynchronization of endogenous circadian pattern of each patient. Diagnosis requires anamnesis, a sleep diary, and actigraphy, if possible; in addition, repeated 24-h measures of circadian markers such as melatonin secretion are also required. Treatment consists of sleep hygiene, behavioral therapy, and melatonin/melatonin agonist administration. Melatonin treatment should start when the circadian rhythm of the patient is in phase with the solar cycle. Efficacy of treatment may be evident after weeks even months from the beginning. There is often a relapse when the medication is stopped.

A Cross-Sectional Study of Circadian Stimulus in Swedish Radiographers' Light Environment

Background: Timely light exposure is a vital aspect to achieve better sleep and well-being. As there are risks with a disturbed circadian rhythm and benefits with light settings that stimulate the rhythm, the circadian effective light, circadian stimulus (CS), for radiographers was examined. Aim: The aim of the study was to compare radiographers' light environment on the workstations, at a university hospital in Southern Sweden in the form of CS and relate that to recommendations published by the Swedish Environmental Protection Agency. Method: A cross-sectional method has been applied. The measurements for CS were collected in all labs in the radiology department in the middle of January. Result: A total of 804 measures were evenly collected resulting in a median for the 19 labs, where the observed median for all labs was 0.091 CS which is significantly lower than the recommended value of 0.3 CS (p < .001). Comparing work light settings with maximum light levels in the brightest and darkest labs showed a significant difference (p < .001). Conclusion: The CS values in the labs, at the radiology department at a university hospital in Southern Sweden, do not reach the recommended values of circadian stimulus published by the Swedish Environmental Protection Agency when the radiographers themselves set the light. There is a potential for improvement as a significant difference could be seen between the chosen level of light and the maximum possible level of light.

Sleep and circadian rhythms modeling: From hypothalamic regulatory networks to cortical dynamics and behavior

Sleep and circadian rhythms are regulated by dynamic physiologic processes that operate across multiple spatial and temporal scales. These include, but are not limited to, genetic oscillators, clearance of waste products from the brain, dynamic interplay among brain regions, and propagation of local dynamics across the cortex. The combination of these processes, modulated by environmental cues, such as light-dark cycles and work schedules, represents a complex multiscale system that regulates sleep-wake cycles and brain dynamics. Physiology-based mathematical models have successfully explained the mechanisms underpinning dynamics at specific scales and are a useful tool to investigate interactions across multiple scales. They can help answer questions such as how do electroencephalographic (EEG) features relate to subthalamic neuron activity? Or how are local cortical dynamics regulated by the homeostatic and circadian mechanisms? In this chapter, we review two types of models that are well-positioned to consider such interactions. Part I of the chapter focuses on the subthalamic sleep regulatory networks and a model of arousal dynamics capable of predicting sleep, circadian rhythms, and cognitive outputs. Part II presents a model of corticothalamic circuits, capable of predicting spatial and temporal EEG features. We then discuss existing approaches and unsolved challenges in developing unified multiscale models.

Irregular sleep-wake rhythm disorder: From the pathophysiologic perspective to the treatment

Irregular sleep-wake rhythm disorder (ISWRD) is an intrinsic circadian rhythm disorder caused by loss of the brain's circadian regulation, through changes of the input and/or output to the suprachiasmatic nucleus (SCN), or of the SCN itself. Although there are limited prevalence data for this rare disease, ISWRD is associated with neurodegenerative disorders, including the Alzheimer disease (AD) and the Parkinson disease (PD), which will become increasingly prevalent in an aging population. It additionally presents in childhood developmental disorders, psychiatric disorders, and traumatic brain injury (TBI). Patients present with unpredictable, short sleep periods over a 24-h period, with significant day-to-day and weekly variability. Symptoms manifest as insomnia and excessive daytime sleepiness. Sleep logs and actigraphy monitoring capture rest-activity patterns required for diagnosis. Treatment aims to enhance external circadian cues through timed light therapy, behavioral activity regimens, and melatonin, but efficacy remains quite limited. Pathophysiology of ISWRD in association with various diseases and their specific management are discussed. There is a need for further investigation of disease pathophysiology, development of widely applicable tools for diagnosis, and development of treatments.

Work Around the Clock: How Work Hours Induce Social Jetlag and Sleep Deficiency

A growing body of evidence has placed an increasing emphasis on how sleep affects health. Not only does insufficient sleep make one subjectively feel worse, but is associated with chronic diseases that are considered epidemics in industrialized nations. This is partly caused by the growing need for prolonged work and social schedules, exemplified by shift work, late-night weekends, and early morning work/school start times (social jetlag). Here, we consider fundamental relationships between the circadian clock and biologic processes and discuss how common practices, such as shift work and social jetlag, contribute to sleep disruption, circadian misalignment, and adverse health outcomes.

Disruptions to sleep and circadian rhythms are associated with poorer athlete mental health in female, but not male, elite Australian Rules footballers

Elite athletes are vulnerable to sleep and circadian disruption and associated mental health symptoms. This study aimed to investigate sex differences in sleep, circadian rhythms, and mental health, as well as the moderating role of sex in the prediction of mental health, among male professional and female semi-professional elite athletes. Participants were 87 elite Australian Rules football (ARF) athletes (43% female; mean [standard deviation] age 24.0 [4.1] years). Participants completed baseline questionnaires, 2 weeks of sleep/wake monitoring via actigraphy, and a circadian phase assessment (dim-light melatonin onset [DLMO]). Cross-sectional data were collected in training-only Australian Football League (AFL) Men's and Women's pre-season periods, with 53 providing data in two pre-seasons. Female athletes, relative to males, reported poorer mental health (a higher athlete psychological strain score), had a later mid-sleep time (by 28 min), reported a greater preference towards eveningness, and displayed a later circadian phase (by 33 min). For female athletes, lower sleep efficiency and lower sleep regularity were associated with poorer mental health. For female athletes, there were U-shaped relationships between both morningness-eveningness and phase angle (interval between sleep onset and DLMO time) and mental health. No significant relationships were found for male athletes. In summary, elite female ARF athletes reported poorer mental health, relative to males, especially when experiencing sleep or circadian disruption. Lifestyle factors associated with sex differences in ARF professionalism (scheduling, finances, supports) may contribute to these findings. Programmes to improve sleep, circadian alignment, and mental health among female semi-professional elite athletes should be strongly considered.

Emerging interactions between circadian rhythms and extracellular vesicles

Circadian rhythms are present across species, tuning internal processes to daily changes in the environment. Driven by genetically encoded circadian clocks present throughout the body, and modulated by external inputs, the circadian system is a key player in metabolic control. However, the molecular mediators underlying coordination between cells and tissues are not well known. Extracellular vesicles (EVs) have emerged over recent years as important players in cell-cell and organ-organ communication, however the influence of circadian rhythms on EVs is not yet understood. Research into this area is still scarce, yet already offers glimpses into the potential impact of circadian rhythms on EV biology. In this review, recent discoveries that reveal, directly or indirectly, a potential role for circadian rhythms in EV abundance, properties, cargo and signalling functions are first discussed. Next, the feedback of EV signalling on circadian clocks is considered. Last, unanswered questions regarding the interaction between circadian rhythms and EVs are examined alongside potential approaches to address them. Overall, the circadian impact on EV signalling is an exciting yet understudied aspect that warrants further investigation.

Mortality risk and burden of sudden cardiac arrest associated with hot nights, heatwaves, cold spells, and non-optimum temperatures in 0.88 million patients: An individual-level case-crossover study

Sudden cardiac arrest (SCA) is a global health concern, imposing a substantial mortality burden. However, the understanding of the impact of various extreme temperature events, when accounting for the effect of daily average temperature on SCA, remains incomplete. Additionally, the assessment of SCA mortality burden associated with temperatures from an individual-level design is limited. This nationwide case-crossover study collected individual SCA death records across all (2844) county-level administrative units in the Chinese Mainland from 2013 to 2019. Four definitions for hot nights and ten for both cold spells and heatwaves were established using various temperature thresholds and durations. Conditional logistic regression models combined with distributed lag nonlinear models were employed to estimate the cumulative exposure-response relationships. Based on 887,662 SCA decedents, this analysis found that both hot nights [odds ratio (OR): 1.28; attributable fraction (AF): 1.32 %] and heatwaves (OR: 1.40; AF: 1.29 %) exhibited significant added effects on SCA mortality independent of daily average temperatures, while cold spells were not associated with an elevated SCA risk after accounting for effects of temperatures. Cold temperatures [below the minimum mortality temperature (MMT)] accounted for a larger mortality burden than high temperatures (above the MMT) [AF: 12.2 % vs. 1.5 %]. Higher temperature-related mortality risks and burdens were observed in patients who experienced out-of-hospital cardiac arrest compared to those with in-hospital cardiac arrest. This nationwide study presents the most compelling and comprehensive evidence of the elevated mortality risk and burden of SCA associated with extreme temperature events and ambient temperatures amid global warming.

Sleep and circadian disruption in bipolar disorders: From psychopathology to digital phenotyping in clinical practice

Sleep and biological rhythms are integral to mood regulation across the lifespan, particularly in bipolar disorder (BD), where alterations in sleep phase, structure, and duration occur in all mood states. These disruptions are linked to poorer quality of life, heightened suicide risk, impaired cognitive function, and increased relapse rates. This review highlights the pathophysiology of sleep disturbances in BD and aims to consolidate understanding and clinical applications of these phenomena. It also summarizes the evolution of sleep and biological rhythms assessment methods, including ecological momentary assessment (EMA) and digital phenotyping. It underscores the importance of recognizing circadian rhythm involvement in mood regulation, suggesting potential therapeutic targets. Future research directions include elucidating circadian clock gene mechanisms, understanding environmental impacts on circadian rhythms, and investigating the bidirectional relationship between sleep disturbances and mood regulation in BD. Standardizing assessment methods and addressing privacy concerns related to EMA technology and digital phenotyping are essential for advancing research. Collaborative efforts are crucial for enhancing clinical applicability and understanding the broader implications of biological rhythms in BD diagnosis and treatment. Overall, recognizing the significance of sleep and biological rhythms in BD offers promise for improved outcomes through targeted interventions and a deeper understanding of the disorder's underlying mechanisms.

Effects of dim-evening lighting optimised for geographical orientation versus standard lighting on mental health: protocol paper for a quasiexperimental study in a psychiatric hospital

INTRODUCTION

Research has provided novel insights into how light stimulates circadian rhythms through specialised retinal ganglion cells to the suprachiasmatic nucleus. In addition, there has been a revolution in light-emitting diode (LED) technology, leading to tunable LED light sources and lighting systems, enabling 24-hour dynamic light scenarios with bright blue-enriched short wavelength light during the day and dim evening light, stimulating the circadian system. These dynamic LED lighting systems are now being implemented at hospitals without adequate understanding of how it may affect the health and well-being of patients and staff.

METHODS AND ANALYSIS

An optimised dynamic LED lighting scenario is investigated at a newly built psychiatric hospital in Copenhagen. In the 12 months baseline period, a standard lighting scenario with dynamic colour temperature and fixed light intensity is investigated. In the following 12-month intervention period, a new DEL scenario is investigated, having dynamic colour temperature as well as dynamic light intensity with a higher daytime and lower evening-time melanopic daylight equivalent illuminance. This setting is furthermore adjusted for geographical orientation to compensate for differences in sunlight access in wintertime. The study uses a quasiexperimental design comparing patients admitted in the two study periods. Prior to each of the study periods, daylight and the contribution from the LED-lighting scenarios was measured. Patient sociodemographic and mental health data will be retrieved retrospectively from electronic medical records and by questionnaires administered in the two periods, evaluating lighting, noise, sleep quality and quality of life. Primary outcome is the proportion of patients receiving pro re nata medications. Secondary outcomes are the length of stay, sleep onset latency, sleep quality and quality of life.

ETHICS AND DISSEMINATION

No ethical issues are expected. The results will be disseminated through peer-reviewed international journal, lectures, posters and interviews.

TRIAL REGISTRATION NUMBER

NCT05868291.

Life onboard a submarine: Sleep, fatigue, and lifestyle behaviors of sailors on a circadian-aligned watchstanding schedule

This study characterizes, for the first time, the lives of U.S. Navy submariners engaged in normal, sea-based operations while following a circadian-aligned 24-h watchstanding schedule. Fifty-eight submarine crewmembers provided objective (actigraphy) and subjective (questionnaires) sleep data, and information about mood and lifestyle behaviors during 30 days underway. Projected performance scores and estimated circadian phase times were also calculated from actigraphy-based sleep/wake data. Submariners' objective (6.62 ± 0.94 h; mean ± SD) and subjective (5.90 ± 1.38 h) daily sleep quantities while underway were largely comparable to the sleep reportedly received by Sailors across other Navy platforms and watchstanding schedules. Additionally, submariners' actigraphy-predicted circadian phases shifted progressively toward better alignment with watchstanding schedules across time. Nevertheless, subjective sleep quality was low, submariners engaged in unfavorable lifestyle behaviors (lack of regular meals and exercise), and participants reported decreased mood at the completion of their underway time. Recommendations for countermeasure development are provided.

The dark side of apnea: altered 24-hour melatonin secretion in obstructive sleep apnea (OSAS) is disease severity dependent

PURPOSE

Melatonin aids in the synchronization of the circadian rhythm to the external environment. Few studies have tried to elucidate the relationship between melatonin and obstructive sleep apnea syndrome (OSAS). These often include few patients, do not differentiate between OSAS severity and/or do not analyse a 24-h melatonin profile. This study set out to investigate disease severity dependent differences in 24-h salivary melatonin secretion of OSAS patients compared to a reference population in a retrospective design.

METHODS

24-h salivary melatonin profiles of 169 OSAS patients were analysed (55 light, 66 moderate, 48 severe) as well as 91 reference patients. Several aspects of the melatonin curve were analysed and stratified according to OSAS severity. Parameters included: dim light melatonin onset (DLMO), time of returning below DLMO (DLMOoff), peak melatonin concentration and time, and total melatonin exposure.

RESULTS

Significant effects were corrected for confounding by age and sex using linear regression. Our analysis shows that, compared to reference and in a disease dependent manner, OSAS patients have a significantly lower 24-h melatonin curve, lower melatonin peak concentration, lower total melatonin exposure and a smaller proportion of patients reach DLMO. The differences in peak melatonin production and total melatonin exposure were resistant to confounding by age and/or sex.

CONCLUSION

This study describes clear OSAS severity dependent abnormalities in melatonin production in OSAS patients, independent of sex and/or age. Future research should indicate whether oral melatonin supplementation has beneficial effects in OSAS patients with attenuated endogenous melatonin production.

Eating Around the Clock: Circadian Rhythms of Eating and Metabolism

The time of day that we eat is increasingly recognized as contributing as importantly to overall health as the amount or quality of the food we eat. The endogenous circadian clock has evolved to promote intake at optimal times when an organism is intended to be awake and active, but electric lights and abundant food allow eating around the clock with deleterious health outcomes. In this review, we highlight literature pertaining to the effects of food timing on health, beginning with animal models and then translation into human experiments. We emphasize the pitfalls and opportunities that technological advances bring in bettering understanding of eating behaviors and their association with health and disease. There is great promise for restricting the timing of food intake both in clinical interventions and in public health campaigns for improving health via nonpharmacological therapies.

Higher central circadian temperature amplitude is associated with greater metabolite rhythmicity in humans

Robust circadian rhythms are essential for optimal health. The central circadian clock controls temperature rhythms, which are known to organize the timing of peripheral circadian rhythms in rodents. In humans, however, it is unknown whether temperature rhythms relate to the organization of circadian rhythms throughout the body. We assessed core body temperature amplitude and the rhythmicity of 929 blood plasma metabolites across a 40-h constant routine protocol, controlling for behavioral and environmental factors that mask endogenous temperature rhythms, in 23 healthy individuals (mean [± SD] age = 25.4 ± 5.7 years, 5 women). Valid core body temperature data were available in 17/23 (mean [± SD] age = 25.6 ± 6.3 years, 1 woman). Individuals with higher core body temperature amplitude had a greater number of metabolites exhibiting circadian rhythms (R2 = 0.37, p = .009). Higher core body temperature amplitude was also associated with less variability in the free-fitted periods of metabolite rhythms within an individual (R2 = 0.47, p = .002). These findings indicate that a more robust central circadian clock is associated with greater organization of circadian metabolite rhythms in humans. Metabolite rhythms may therefore provide a window into the strength of the central circadian clock.

Circadian photoentrainment varies by season and depressed state: associations between light sensitivity and sleep and circadian timing

STUDY OBJECTIVES

Altered light sensitivity may be an underlying vulnerability for disrupted circadian photoentrainment. The photic information necessary for circadian photoentrainment is sent to the circadian clock from melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs). The current study tested whether the responsivity of ipRGCs measured using the post-illumination pupil response (PIPR) was associated with circadian phase, sleep timing, and circadian alignment, and if these relationships varied by season or depression severity.

METHODS

Adult participants (N = 323, agem = 40.5, agesd = 13.5) with varying depression severity were recruited during the summer (n = 154) and winter (n = 169) months. Light sensitivity was measured using the PIPR. Circadian phase was assessed using Dim Light Melatonin Onset (DLMO) on Friday evenings. Midsleep was measured using actigraphy. Circadian alignment was calculated as the DLMO-midsleep phase angle. Multilevel regression models covaried for age, gender, and time since wake of PIPR assessment.

RESULTS

Greater light sensitivity was associated with later circadian phase in summer but not in winter (β = 0.23; p = 0.03). Greater light sensitivity was associated with shorter DLMO-midsleep phase angles (β = 0.20; p = 0.03) in minimal depression but not in moderate depression (SIGHSAD < 6.6; Johnson-Neyman region of significance).

CONCLUSIONS

Light sensitivity measured by the PIPR was associated with circadian phase during the summer but not in winter, suggesting ipRGC functioning in humans may affect circadian entrainment when external zeitgebers are robust. Light sensitivity was associated with circadian alignment only in participants with minimal depression, suggesting circadian photoentrainment, a possible driver of mood, may be decreased in depression year-round, similar to decreased photoentrainment in winter.

Sex differences in sleep, circadian rhythms, and metabolism: Implications for precision medicine

The number of individuals experiencing sleep loss has exponentially risen over the past decades. Extrapolation of laboratory findings to the real world suggests that females are more affected by extended wakefulness and circadian misalignment than males are. Therefore, long-term effects such as sleep and metabolic disorders are likely to be more prevalent in females than in males. Despite emerging evidence for sex differences in key aspects of sleep-wake and circadian regulation, much remains unknown, as females are often underrepresented in sleep and circadian research. This narrative review aims at highlighting 1) how sex differences systematically impinge on the sleep-wake and circadian regulation in humans, 2) how sex differences in sleep and circadian factors modulate metabolic control, and 3) the relevance of these differences for precision medicine. Ultimately, the findings justify factoring in sex differences when optimizing individually targeted sleep and circadian interventions in humans.

The environment and the internal clocks: The study of their relationships from prehistoric to modern times

The origin of biological rhythms goes back to the very beginning of life. They are observed in the animal and plant world at all levels of organization, from cells to ecosystems. As early as the 18th century, plant scientists were the first to explain the relationship between flowering cycles and environmental cycles, emphasizing the importance of daily light-dark cycles and the seasons. Our temporal structure is controlled by external and internal rhythmic signals. Light is the main synchronizer of the circadian system, as daily exposure to light entrains our clock over 24 hours, the endogenous period of the circadian system being close to, but not exactly, 24 hours. In 1960, a seminal scientific meeting, the Cold Spring Harbor Symposium on Biological Rhythms, brought together all the biological rhythms scientists of the time, a number of whom are considered the founders of modern chronobiology. All aspects of biological rhythms were addressed, from the properties of circadian rhythms to their practical and ecological aspects. Birth of chronobiology dates from this period, with the definition of its vocabulary and specificities in metabolism, photoperiodism, animal physiology, etc. At around the same time, and right up to the present day, research has focused on melatonin, the circadian neurohormone of the pineal gland, with data on its pattern, metabolism, control by light and clinical applications. However, light has a double face, as it has positive effects as a circadian clock entraining agent, but also deleterious effects, as it can lead to chronodisruption when exposed chronically at night, which can increase the risk of cancer and other diseases. Finally, research over the past few decades has unraveled the anatomical location of circadian clocks and their cellular and molecular mechanisms. This recent research has in turn allowed us to explain how circadian rhythms control physiology and health.

A circadian clock regulates the blood-brain barrier across phylogeny

As the central regulatory system of an organism, the brain is responsible for overseeing a wide variety of physiological processes essential for an organism's survival. To maintain the environment necessary for neurons to function, the brain requires highly selective uptake and elimination of specific molecules through the blood-brain barrier (BBB). As an organism's activities vary throughout the day, how does the BBB adapt to meet the changing needs of the brain? A mechanism is through temporal regulation of BBB permeability via its circadian clock, which will be the focal point of this chapter. To comprehend the circadian clock's role within the BBB, we will first examine the anatomy of the BBB and the transport mechanisms enabling it to fulfill its role as a restrictive barrier. Next, we will define the circadian clock, and the discussion will encompass an introduction to circadian rhythms, the Transcription-Translation Feedback Loop (TTFL) as the mechanistic basis of circadian timekeeping, and the organization of tissue clocks found in organisms. Then, we will cover the role of the circadian rhythms in regulating the cellular mechanisms and functions of the BBB. We discuss the implications of this regulation in influencing sleep behavior, the progression of neurodegenerative diseases, and finally drug delivery for treatment of neurological diseases.

Behavioral-Social Rhythms and Cardiovascular Disease Risk in Retired Night Shift Workers and Retired Day Workers

OBJECTIVE

Stability in the timing of key daily routine behaviors such as working/doing housework, sleeping, eating, and engaging in social interactions (i.e., behavioral-social rhythms) contributes to health. This study examined whether behavioral-social rhythms were associated with cardiovascular disease (CVD) risk factors in retired night shift workers and retired day workers and explored whether past night shift work exposure moderated this association.

METHODS

A total of 154 retired older adults participated in this study. Multiple logistic regression models were used to examine associations between behavioral-social rhythms and CVD risk factors. Independent variables included Social Rhythm Metric (SRM)-5 score and actigraphy rest-activity rhythm intradaily variability (IV) and interdaily stability (IS). Dependent variables were metabolic syndrome prevalence and its five individual components.

RESULTS

More regular behavioral-social rhythms were associated with lower odds of prevalent metabolic syndrome (SRM: odds ratio [OR] = 0.57, 95% confidence interval [CI] = 0.35-0.88; IV: OR = 4.00, 95% CI = 1.86-8.58; IS: OR = 0.42, 95% CI = 0.24-0.73) and two of its individual components: body mass index (SRM: OR = 0.56, 95% CI = 0.37-0.85; IV: OR = 2.84, 95% CI = 1.59-5.07; IS: OR = 0.42, 95% CI = 0.26-0.68) and high-density lipoprotein cholesterol (SRM: OR = 0.49, 95% CI = 0.30-0.80; IV: OR = 2.49, 95% CI = 1.25-4.96; IS: OR = 0.35, 95% CI = 0.19-0.66). Past shift work history did not moderate the association between behavioral-social rhythms and metabolic syndrome.

CONCLUSIONS

Behavioral-social rhythms were related to CVD risk factors in retired adults regardless of prior night shift work exposure. Older retired workers may benefit from education and interventions aiming to increase behavioral-social rhythm regularity.

Lengthened circadian rhythms in mice with self-controlled ambient light intensity

Laboratory animals are typically maintained under 12-h light and 12-h dark (12:12 LD) conditions with a daytime light intensity of ~ 200 lx. In this study, we designed an apparatus that allowed mice to self-select the room light intensity by nose poking. We measured the behavioral rhythms of the mice under this self-controlled light regimen. The mice quickly learned the relationship between their nose pokes and the resulting changes in the light intensity. Under these conditions, the mice exhibited free-running circadian behavior with a period of 24.5 ± 0.4 h. This circadian period was ~ 1 h longer than that of the same strain of mice when they were kept in constant darkness (DD) after 12:12 LD entrainment, and the lengthened period lasted for at least 30 days. The rhythm of the light intensity controlled by the mice also exhibited a similar period, but the phase of the illuminance rhythm preceded the phase of the locomotor activity rhythm. Mice that did not have access to the light controller were also entrained to the illuminance cycle produced by the mice that did have access to the light controller, but with a slightly delayed phase. The rhythm was likely controlled by the canonical circadian clock because mice with tau mutations in the circadian clock gene CSNK1E exhibited short periods of circadian rhythm under the same conditions. These results indicate that the free-running period of mice in the wild may differ from what they exhibit if they are attuned by forced light cycles in laboratories because mice in their natural habitats can self-control their exposure to ambient light, similar to our experimental conditions.

Mathematical Analysis of Light-sensitivity Related Challenges in Assessment of the Intrinsic Period of the Human Circadian Pacemaker

Accurate assessment of the intrinsic period of the human circadian pacemaker is essential for a quantitative understanding of how our circadian rhythms are synchronized to exposure to natural and man-made light-dark (LD) cycles. The gold standard method for assessing intrinsic period in humans is forced desynchrony (FD) which assumes that the confounding effect of lights-on assessment of intrinsic period is removed by scheduling sleep-wake and associated dim LD cycles to periods outside the range of entrainment of the circadian pacemaker. However, the observation that the mean period of free-running blind people is longer than the mean period of sighted people assessed by FD (24.50 ± 0.17 h vs 24.15 ± 0.20 h, p < 0.001) appears inconsistent with this assertion. Here, we present a mathematical analysis using a simple parametric model of the circadian pacemaker with a sinusoidal velocity response curve (VRC) describing the effect of light on the speed of the oscillator. The analysis shows that the shorter period in FD may be explained by exquisite sensitivity of the human circadian pacemaker to low light intensities and a VRC with a larger advance region than delay region. The main implication of this analysis, which generates new and testable predictions, is that current quantitative models for predicting how light exposure affects entrainment of the human circadian system may not accurately capture the effect of dim light. The mathematical analysis generates new predictions which can be tested in laboratory experiments. These findings have implications for managing healthy entrainment of human circadian clocks in societies with abundant access to light sources with powerful biological effects.

Spatiotemporal light exposure modeling for environmental circadian misalignment and solar jetlag

Background

Light exposure is the most powerful resetting signal for circadian rhythms. The objective of this study was to develop and validate a high-resolution geospatial light exposure model that measures environmental circadian misalignment (or solar jetlag) as the mismatch between the social clock and sun clock, which occurs from geographic variation in light exposure leading to delayed circadian phase from relatively less morning light exposure and greater evening light exposure with increasing westward position within a time zone.

Methods

The light exposure model (30 m2 spatial resolution) incorporated geospatial data across the United States on time zones, elevation (using Google Earth Engine), sunrise time, and sunset time to estimate solar jetlag scores (higher values indicate higher environmental circadian misalignment). The validation study compared the light exposure model in 2022, which was linked with geocoded residential addresses of n = 20 participants in Boston, MA (eastern time zone position) and Seattle, WA (western time zone position) using a geographic information system, with illuminance values captured from wearable LYS light sensors and with sun times from the Solar Calculator.

Results

Western versus eastern positions within a time zone were associated with higher solar jetlag scores from the light exposure model (P < 0.01) and relatively larger differences in sunset time measured using light sensors (social clock) and the Solar Calculator (sun clock) (P = 0.04).

Conclusion

We developed and validated a geospatial light exposure model, enabling high spatiotemporal resolution and comprehensive characterization of geographic variation in light exposure potentially impacting circadian phase in epidemiologic studies.

Estimating dim light melatonin onset time in children using delta changes in melatonin

We aimed to establish a method for estimating dim light melatonin onset (DLMO) using mathematical slopes calculated from melatonin concentrations at three sampling points before and after sleep in children. The saliva of 30 children (mean age ± SD: 10.2 ± 1.3 years old) was collected under dim-light conditions up to six times every hour starting at 17:30 (t17), namely, 18:30 (t18), 19:30 (t19), 20:30 (t20), 21:30 (t21), 22:30 (t22), and 23:30 (t23), in the evening, and at 6:00 (t30) the following morning. We calculated SLOPE on (mathematical slope between melatonin concentrations at t18 and t20, t21 or t22), SLOPE off (the slope between t20, t21 or t22, and t30), and Δ S L O P E , which is generated by subtracting SLOPE on from SLOPE off . DLMO was estimated by multiple regression analysis with the leave-one-out cross-validation (LOOCV) method using SLOPE on and SLOPE off , and Δ S L O P E . The intraclass correlation coefficient (ICC) between the estimated and measured DLMOs was used as the index of estimation accuracy. DLMOs estimated using multiple regression equations with SLOPE on and SLOPE off yielded significant ICCs for the measured DLMOs, with the largest ICC at t20 (ICC = 0.634). Additionally, the ICC between the estimated and measured DLMOs using the equation with Δ S L O P E was significant, with a larger ICC at t20 (ICC = 0.726) than that of the equation with SLOPE on and SLOPE off . Our results showed that DLMO could be estimated with a certain level of accuracy from salivary melatonin levels at three time points before and after sleep in children.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41105-023-00493-x.

Normal Sleep in Children and Adolescence

Adequate sleep is essential for healthy development in childhood and adolescence. Healthy sleep contributes to good physical health, immune function, mental health, and academic performance. The regulation and architecture of sleep change greatly across childhood and adolescence, and the ability to obtain sufficient sleep is impacted by a range of factors that change with maturation. This article describes normal sleep across childhood and adolescence and discusses some of the most common barriers to adequate sleep, including early school start times, technology use, and changes to circadian rhythms, and sleep homeostasis across puberty.