Journal of Biological Rhythms

BackgroundShift work, or working outside of normal circadian cycles, is associated with both experience of fatigue and poorer long-term physical and mental health. However, current methods for assessing fatigue present several barriers to building understanding of how these risks develop over time within individuals. ObjectiveHere, we explore the potential of momentary speech activity-based fatigue measurement in a large, multi-lingual cohort of shift workers, using an intensive longitudinal study design (twice-daily measurement over two weeks in N=1,197 individuals from six different countries). MethodsParalinguistic speech features were used to predict different aspects of acute and chronic fatigue at each study time-point, with performance assessed in unseen (held-out) data. Results are reported both across the dataset as a whole, and for user-specific prediction models. FindingsIn the cross-sectional analysis, good (close to or exceeding current state-of-the-art) performance was achieved for both current sleep deprivation and self-reported sleepiness levels. The within-user analysis revealed robust increases in performance, yielding the ability to detect more subjective aspects of fatigue such as pervasive physical and mental exhaustion. ConclusionsThese findings offer preliminary support for the utility of using brief momentary speech samples as a low-burden, acceptable, and reliable measure of different aspects of fatigue in high-risk populations such as shift workers. Clinical ImplicationsDeveloping brief, accessible measures of different dimensions of fatigue is an important step towards building understanding of how risks for poorer health outcomes develop over time within individuals exposed to significant circadian disruption. SummaryO_ST_ABSWhat is already known on this topicC_ST_ABSPrevious studies have indicated that speech data may be a promising source of information about fatigue: however to date these have primarily been carried out in small, unrepresentative samples and in non-naturalistic settings, making results hard to generalize. What this study addsHere, we present evidence from a large international study where participants both provided momentary speech activity data and reported levels of different aspects of fatigue, as they went about their usual working lives. Importantly, we use a modelling framework that explicitly takes into account potential influences of factors such as age, sex, and language on speech features and target fatigue measures, as well as assessing potential biases and reliability of model output. How this study might affect research, practice or policyIf individuals who are at heightened risk of fatigue-related health problems are able to monitor their fatigue levels regularly and in real-time, this may yield the opportunity to intervene prior to transitioning into less tractable states of poor physical and mental health.

Both active movement profiles and robust circadian rhythms are linked to improved health outcomes, yet the underlying mechanisms remain partially understood. We investigated this relationship in young adults (n = 169, aged 18-30 years) under real-world conditions using actigraphy data. We performed k-means clustering on 12 accelerometer-based features capturing magnitude, duration, frequency, and intensity distribution to derive movement behavior profiles. As a proxy of circadian rhythms integrity we computed the Circadian Function Index (CFI), which combines intradaily variability, interdaily stability, and relative amplitude. We also assessed circadian phase and sleep quality parameters. Additionally, we quantified light exposure and physical activity over 3-hour daily intervals. The unsupervised algorithm identified two non-overlapping profiles among participants, the More Active (MA) and the Less Active (LA) profiles. MA exhibited a higher CFI (0.81 {+/-} 0.06 vs. 0.69 {+/-} 0.06, p <0.001), which was also positively associated with early-evening physical activity, but not with light exposure. MA also showed an earlier circadian phase, estimated as the midpoint of the five least active hours (L5c, 04:30 {+/-} 01:03 vs. 04:59 {+/-} 01:15, p adj. = 0.04), which was inversely associated with early-morning physical activity and late-morning light exposure. We found no differences in sleep quality between MA and LA. Our results underscore the association between movement behavior and overall circadian rhythms integrity. Importantly, these findings reinforce actigraphy as a multidimensional tool for both health research and clinical applications.

Identifying behavioral markers of disease risk associated with circadian disruption remains challenging due to reproducibility concerns, as human studies are often under-powered to detect the effects of interest. Submarines provide an extreme example of cir-cadian disruption, depriving individuals of natural light-dark cycles and limiting oppor-tunities for physical activity. To address these challenges, we developed an effect size-based method to robustly screen potential actigraphy-derived markers of the submarine environment. Actigraphy data was collected across the pre-mission, mission and post-mission periods. During the mission, crew members followed a 6-hours-on/6-hours-off split shift schedule, except for submariners with on-call duty or extended hours. Forty-one actigraphy variables were screened for their ability to differentiate between study periods and shifts. A Monte Carlo permutation test was used for this initial screening, followed by receiver operating characteristic (ROC) curve analysis for pairwise compar-isons. Light-related features, the slopes of regression lines fitted to the daily predicted dim light melatonin onset (DLMO) trajectories, and Pearson correlation coefficients be-tween each recording day and its corresponding predicted DLMO demonstrated the great-est potential to distinguish between study periods. Most high-performing features dis-criminated between split shift work schedules and on-call or extended-hours schedules. Only the relative amplitude (RA) and the average light intensity during the five least-illuminated hours of the day (L5) reliably distinguished between the shift performing the bulk of nighttime work and its counterpart. Although submarine settings offer an oppor-tunity to study a healthy population living under confined movement and dim light, de-tecting biological signals through actigraphy in this environment is inherently challeng-ing. Demonstrating robust effects under these conditions supports the use of actigraphy in dimly-lit, isolated, confined, and extreme (DICE) environments, but also its applica-bility to study other operational or clinical populations with reduced activity levels.

Many genes exhibit circadian rhythms in expression. The amplitude of oscillation, both in core clock and circadian output genes, may differ from person to person. Mutations in core clock genes are known to alter global rhythmic properties, and researchers often informally discuss "circadian amplitude." Yet, it remains unclear whether, in the general population, differences in transcriptional amplitude are largely gene-specific, or if they reflect a global, transcriptome-wide pattern -- whether some individuals have globally higher or lower amplitude across the set of all rhythmic genes. We used Cosinor regression to reanalyze four human skin time-series transcriptomic datasets (paired epidermis/dermis samples, N=11, N=19) and found that, using either absolute or relative amplitude measures, distributions of gene amplitudes tended to cluster by subject. Using a non-parametric, permutation-based statistical test, we found that in many subjects this global amplitude trend was statistically significant (p [&le;] 0.01). Furthermore, we found that when rhythmic genes were divided into two sets based on peak time (genes peaking before-noon and after-noon), the subjects global amplitude in one set predicted global amplitude in the other set (p [&le;] 0.05). We also found that in the paired epidermis/dermis datasets, subjects global amplitude in epidermis predicted their global amplitude in the dermis (p [&le;] 0.05). After identifying these trends in the skin datasets, we then found that evidence for subject-specific transcriptional rhythm strength replicated across 6 additional human time-course datasets from adipose, muscle, and blood. Perhaps surprisingly, across datasets, we found that neither established metrics of core clock transcriptional organization nor the amplitude of core clock transcription were strongly correlated with subject-specific global amplitude.

BACKGROUNDTwenty-four-hour rhythmicity in transcriptomes of tissues and organs is driven by local circadian oscillators, systemic factors, the central circadian pacemaker, and light-dark cycles. This rhythmicity is to some extent organ- and tissue-specific such that the sets of rhythmic transcripts or their timing are different across tissues/organs. Monitoring rhythmicity of tissues and organs holds promise for circadian medicine, but in humans most tissues and organs are not easily accessible. To investigate the extent to which rhythmicity in the human blood transcriptome reflects rhythmicity in tissues and organs, we compared the overlap and timing of rhythmic transcripts in human blood and rhythmic transcripts in 64 tissues/organs of the baboon. METHODSRhythmicity in the transcriptomes of humans and baboons were compared using set logic, circular cross-correlation, circular clustering, functional enrichment analyses and partial least squares regression. RESULTSOf the 759 orthologous genes that were rhythmic in human blood, 652 (86%) were also rhythmic in at least one baboon tissue. Most of these genes were associated with basic processes such as transcription and protein homeostasis. 109 (17%) of the 652 overlapping rhythmic genes were reported as rhythmic in only one baboon tissue or organ and several of these genes have tissue/organ-specific functions. Analysis of the alignment between baboon and human transcriptomes showed that in these diurnal species, rhythmicity is aligned with the onset, rather than midpoint or end of light period. In both species, the timing of rhythmic transcripts displayed prominent night and day clusters, with genes in the dark cluster associated with translation. The timing of human and baboon transcriptomes was significantly correlated in 25 tissue/organs with an average earlier timing of 3.21 h (SD 2.47 h) in human blood. CONCLUSIONSThe human blood transcriptome contains sets of rhythmic genes that overlap with rhythmic genes of tissues/organs, some of which are tissue/organ-specific, in the baboon. The rhythmic sets vary across tissues/organs but the timing of most rhythmic genes is similar across human blood and baboon tissues/organs. These results have implications for our understanding of the regulation of rhythmicity across tissues/organs and species and development of blood transcriptome-based biomarkers for rhythmicity in tissues and organs.

The problem of entrainment is central to circadian biology. In this regard, Drosophila has been an important model system. Owing to the simplicity of its nervous system and the availability of powerful genetic tools, the system has shed significant light on the molecular and neural underpinnings of entrainment. However, much remains to be learned regarding the molecular and physiological mechanisms underlying this important phenomenon. Under cyclic light/dark conditions, Drosophila melanogaster displays crepuscular patterns of locomotor activity with one peak anticipating dawn and the other anticipating dusk. These peaks are characterized through an estimation of their phase relative to the environmental light cycle and the extent of their anticipation of light transitions. In Drosophila chronobiology, estimations of phases are often subjective, and anticipation indices vary significantly between studies. Though there is increasing interest in building flexible analysis software tools in the field, none incorporates objective measures of Drosophila activity peaks in combination with the analysis of fly activity/sleep in the same program. To this end, we have developed PHASE, a MATLAB-based program that is simple and easy to use and (i) supports the visualization and analysis of activity and sleep under entrainment, (ii) allows analysis of both activity and sleep parameters within user-defined windows within a diurnal cycle, (iii) uses a smoothing filter for the objective identification of peaks of activity (and therefore can be used to quantitatively characterize them), and (iv) offers a series of analyses for the assessment of behavioral anticipation of environmental transitions.

Study objectives: Light, acting primarily via melanopsin-mediated signaling, plays a central role in synchronising circadian rhythms. Individuals vary markedly in the sensitivity of their circadian system to light. Whether these differences contribute to the interindividual variability in chronotype, a behavioural manifestation of internal circadian timing, is unclear. The aim of this study was to determine the relationship between melanopsin-dependent light sensitivity and chronotype across the general population. Methods: Participants (adults and children aged [&ge;] 8 years) were recruited in a science museum. Chronotype was determined using the {micro}MCTQ and the post-illumination pupillary response (PIPR) to short and long-wavelength light stimuli was used as a measure of melanopsin-dependent light sensitivity. The relationship between PIPR and chronotype and their interaction with age and sex were assessed using multiple linear regression. Results: Pupil recordings and questionnaires were available from 457 participants, including 284 adults and 173 children. In adults, the relationship between melanopsin-dependent light sensitivity and chronotype depends on sex and age: in young adult men, greater light sensitivity is linked to a significantly later chronotype, whereas it is significantly associated with an earlier chronotype in older adult women. In children, no evidence was found for a relationship between light sensitivity and chronotype. Conclusions: These findings suggest that individual variation in light sensitivity interacts with sex and age-specific differences in the circadian system and light exposure behaviour to influence circadian timing. Light exposure recommendations should be personalised to take into account these sex and age-specific effects. Statement of significanceWhile individuals differ widely in how their circadian system responds to light, to what extent these individual responses influence internal circadian timing remains unclear. By studying a large and diverse sample of children and adults, our findings reveal that the relationship between light sensitivity and chronotype, a behavioural manifestation of circadian timing, is shaped by both age and sex, offering a more comprehensive understanding of this relationship than previously recognised. Specifically, greater light sensitivity is linked to later chronotype in young men but to earlier chronotype in older women. These results reveal that the impact of light on circadian timing changes across the lifespan and will contribute to the development of personalised light exposure guidelines to promote health and well-being.

BackgroundCircadian rhythms time physiological and behavioral processes to 24-hour cycles. It is generally assumed that most cells contain self-sustained circadian clocks that drive circadian rhythms in gene expression that ultimately generating circadian rhythms in physiology. While those clocks supposedly act cell autonomously, current work suggests that in Drosophila some of them can be adjusted by the brain circadian pacemaker through neuropeptides, like the Pigment Dispersing Factor (PDF). Despite these findings and the ample knowledge of the molecular clockwork, it is still unknown how circadian gene expression in Drosophila is achieved across the body. ResultsHere, we used single-cell and bulk RNAseq data to identify cells within the fly that express core-clock components. Surprisingly, we found that less than a third of the cell types in the fly express core-clock genes. Moreover, we identified Lamina wild field (Lawf) and Ponx-neuro positive (Poxn) neurons as putative new circadian neurons. In addition, we found several cell types that do not express core clock components but are highly enriched for cyclically expressed mRNAs. Strikingly, these cell types express the PDF receptor (Pdfr), suggesting that PDF drives rhythmic gene expression in many cell types in flies. Other cell types express both core circadian clock components and Pdfr, suggesting that in these cells, PDF regulates the phase of rhythmic gene expression. ConclusionsTogether, our data suggest three different mechanisms generate cyclic daily gene expression in cells and tissues: canonical endogenous canonical molecular clock, PDF signaling-driven expression, or a combination of both.

Light induced improvements in alertness are more prominent during night-time than during the day, indicating circadian regulation or wake duration related dependence. Relative contributions of both factors can be quantified using a forced desynchrony (FD) designs. Here we investigate alerting effects of light in a novel 4x18 hours FD protocol (5h sleep, 13h wake) under dim (6 mlux) and bright light (1159 mlux) conditions. Hourly saliva samples (melatonin and cortisol assessment) and 2-hourly test-sessions were used to assess effects of bright light on subjective and objective alertness (electroencephalography and performance). Results reveal (1) stable free-running cortisol rhythms with uniform phase progression under both light conditions, indicating that FD designs can be conducted under high intensity lighting, (2) subjective alerting effects of light depend on elapsed time awake, while (3) light consistently improves objective alertness independent of time awake. Three dimensional graphs reflecting light induced alertness improvements depending on wake duration related variation and circadian clock phase suggest that performance is improved during daytime, while subjective alertness remains unchanged. This suggests that light during office hours might be beneficial for performance, even though this may not be perceived as such.

Many environmental features are cyclic, with predictable daily and yearly changes which vary across latitudes. Organisms cope with such change using internal timekeepers or circadian clocks which have evolved remarkable flexibility. This flexibility is evident in the waveforms of behavioural and underlying molecular rhythms. In todays world, many ecosystems experience artificial light at night, leading to unusual photoperiodic conditions. Additionally, occupational demands expose many humans to unconventional light cycles. Yet, practical means of manipulating activity waveforms for beneficial purposes are lacking. This requires an understanding of principles and factors governing waveform plasticity of activity rhythms. Even though waveform plasticity remains underexplored, few recent studies have used novel light regimes, inspired by shift work schedules, with alternating bright light and dim light (LDimLDim) to manipulate the activity waveform of nocturnal rodents. We undertook this study to understand what aspects of light regimes contribute to waveform flexibility and how the underlying neuronal circuitry regulates the behaviour by subjecting Drosophila melanogaster to novel light regimes. Using a range of LDimLDim regimes, we found that dim scotopic illumination of specific durations induces activity bifurcation in fruit flies, similar to mammals. Thus, we suggest evolutionarily conserved effects of features of the light regime on waveform plasticity. Further, we demonstrate that the circadian photoreceptor CRYPTOCHROME is necessary for activity bifurcation. We also find evidence for circadian reorganisation of the pacemaker circuit wherein the evening neurons regulate the timing of both bouts of activity under novel light regimes. Thus, such light regimes can be explored further to understand the dynamics and coupling within the circadian circuit. The conserved effects of specific features of the light regime open up the possibility of designing other regimes to test their physiological impact and leverage them for waveform manipulation to minimise the ill effects of unusual light regimes. Author SummaryIt is thought that the appropriate timing of physiological and behavioural rhythms of organisms with respect to the environmental cycle confers an adaptive advantage. Endogenous timekeepers or circadian clocks regulate such rhythms. To optimally time biological rhythms, its waveform must be plastic and respond to changes in external cycles. Changes in external cycles may be natural, as seen across latitudes or seasons, or anthropogenic, such as artificial light induced changes in photoperiod or shiftwork driven novel light/dark cycles. Previous studies using a nocturnal rodent model showed that novel light regimes (LDimLDim) caused locomotor activity to bifurcate such that mice showed two bouts of activity restricted to the dimly lit phases. Here, we first demonstrate that conserved features of the light regime - dim scotopic illumination of specific light durations induce activity bifurcation in the fly model. We leverage the genetic toolkit of the Drosophila model to also show evidence for the reorganisation of the circadian pacemaker neuronal network upon exposure to novel light regimes. Our findings indicate that conserved effects of specific features of the environmental regimes can be exploited to design light regimes that ease the waveform into synchronising with challenging conditions such as during shift work, jetlag, and photoperiodic changes.

BackgroundMental health disorders affect over 1 billion people globally, with circadian rhythm disruption emerging as a key pathophysiological mechanism. Digital biomarkers offer potential for scalable assessment, but validation remains limited. ObjectiveTo develop and validate a Bioelectric Coherence Index (BCI) using smartphone sensors for circadian dysfunction assessment across 50 mental health conditions through digital twin simulation. MethodsWe developed a BCI using smartphone sensors (light exposure, sleep patterns, pupillometry) with the formula: BCI(t) = {middle dot}{Phi}(t) + {beta}{middle dot}{Delta}(t) - {gamma}{middle dot}{Psi}(t), integrating phase alignment, circadian amplitude, and temporal variability components. We simulated 1,000 virtual subjects (20 per condition) across 50 mental health conditions over 365 days, stratified by circadian impact tier. ResultsDigital twin validation achieved 71.2% diagnostic accuracy (95% CI: 68.8-73.6%) across all 50 conditions. Performance varied by circadian impact tier: Core Circadian conditions (87.3%), High Circadian (76.5%), Moderate Circadian (65.2%), and Lower Circadian (52.8%). Cross-validation demonstrated robust performance with minimal overfitting. External benchmarking against published chronotherapy literature showed strong correlations (r = 0.84, p < 0.001). ConclusionsThis digital twin validation provides preliminary computational evidence for smartphone-based circadian assessment utility across diverse mental health conditions. The tier-based performance hierarchy aligns with circadian biology principles. Clinical validation through prospective trials remains essential before clinical application.

Circadian clocks govern daily physiological and behavioral processes and are crucial for health, yet disruptions can lead to various diseases. Chronotype, the state of circadian timing, varies between individuals and is reflected in behaviors such as sleep-wake patterns, cognitive performance, and physical activity. This interindividual variability is influenced by both genetic factors and environmental cues, but the relative contributions of each remain unclear, particularly in terms of plasticity - how much chronotype can shift in response to lifestyle and environmental factors. The gold standard for chronotype assessment, dim-light melatonin onset (DLMO), is invasive and impractical for large-scale studies, while blood-based molecular biomarker tests, which estimate internal time, show promise but are limited by practicality. Here, we introduce HairTime, a novel assay that estimates chronotype from a single hair sample collected at one point during the day. HairTime was developed and evaluated in two studies: a training study and a validation study, where it demonstrated a strong ability to predict chronotype, with DLMO as the comparison. This non-invasive method is suitable for large-scale, longitudinal studies and clinical practice. We assessed HairTime using over 4,000 samples, observing a normal distribution of chronotype across the population, with its estimation associating with age, sex, and notably, work schedules. The association with work schedules reveals the plasticity of chronotype, as workdays circadian timing earlier, highlighting that societal factors can influence and modify an individuals internal rhythm. Additionally, we explore the concept of circadian amplitude, finding that lower amplitude rhythms in hair follicle cells are linked to reduced chronotype prediction accuracy. Our results highlight that both intrinsic circadian mechanisms and external factors, such as lifestyle and work schedules, shape chronotype. HairTime offers an innovative tool for understanding circadian rhythms, facilitating personalized chronotherapy to improve health outcomes by aligning treatments with an individuals biological rhythms.

A wide range of comorbidities have been observed with neuropsychiatric disorders, of which sleep disturbances are one of the most common. Chronotype, a self-reported measurement of an individuals preference for earlier or later sleep timing, is a proxy sleep measure that has been linked to neuropsychiatric disorders (NPDs). By investigating how chronotype influences risk for neuropsychiatric disorders and, vice versa, how risk for neuropsychiatric disorders influences chronotype, we may identify modifiable risk factors for each phenotype. By investigating the specific genetic mechanisms that are common to the risk of evening chronotype and the risk of NPDs, we may gain further understanding of the relationship and causal direction in these phenotypes. Here we use Mendelian randomisation (MR), a method used to explore causal effects, to 1) study the causal relationships between neuropsychiatric disorders and chronotype and 2) characterise the genetic components of these phenotypes. Firstly, we investigated if a causal role exists between six neuropsychiatric disorders and chronotype using the largest genome-wide association studies (GWAS) available. Secondly, we integrated data from expression quantitative trait loci (eQTLs) to investigate the role of gene expression alterations on these phenotypes. We also used colocalization to validate that the same variant is causal for gene expression and each outcome. We identified that the evening chronotype is causal for increased risk of schizophrenia and autism spectrum disorder and, in the opposite direction, that insomnia and schizophrenia are causal for a tendency towards evening chronotype. We identified twelve eQTLs where gene expression changes in brain or blood were causal for one of the tested phenotypes (bipolar disorder, chronotype and schizophrenia). These findings provide important evidence for the complex, bidirectional relationship that exists between these sleep and neuropsychiatric disorders, and use gene expression data to identify causal roles for genes at associated loci. Author SummarySleep disturbances are commonly observed features of neuropsychiatric disorders. Chronotype, a behavioural manifestation of an individuals preference for early or late sleep timing i.e. morning chronotype means a preference for earlier sleep and wake times, has been used as a behavioural marker of underlying circadian function. Here, we used data from the largest genetic studies available to test the causal relationship between chronotype and risk for neuropsychiatric disorders. We found that individuals with the evening chronotype have greater risk for schizophrenia or autism spectrum disorder. In the other direction, we found insomnia or schizophrenia diagnosis is causal for a tendency towards evening chronotype. We searched for DNA variants that influence chronotype or risk for neuropsychiatric disorders through alterations of gene expression in blood and brain tissues. We found twelve DNA variants with a significant effect on chronotype or risk of either bipolar disorder or schizophrenia. These results demonstrate that sleep and neuropsychiatric disorders have a complex bidirectional relationship and that the causal role of some genes is due to variants that alter gene expression.

IntroductionWearable accelerometers are a valuable tool for monitoring sleep, sedentary behaviour, and physical activity patterns within 24h time-use in free-living environments. While wrist-worn accelerometers are favoured for monitoring sleep, they do not accurately distinguish between sitting and lying positions (Narayanan et al., 2020). This study aims to determine whether back or thigh-mounted accelerometers yield sleep metrics comparable to wrist-worn devices using an open-source algorithm originally validated for the wrist. MethodsData from 20 healthy sleepers were collected using Axivity AX3 accelerometers. Participants wore accelerometers on their right thigh, low-back, and wrist for one night of sleep in their own bed. Sleep metrics were calculated using the van Hees algorithm through the GGIR package in R. The primary outcomes were: Total Sleep Time (TST), Wake After Sleep Onset (WASO), Awakenings (AWK), Sleep Efficiency (SE), Sleep Interval (SI) and Sleep Onset Timestamp (SOT). Within-subject ANOVA with Tukeys post hoc, Pearson correlation coefficients, Bland-Altman plots, and Cohens d were used to assess the comparability of sleep metrics between the body placements. ResultsData analysis included all 20 participants. Mid-thigh accelerometers demonstrated a strong linear relationship with wrist accelerometers across all metrics (r = 0.86-0.98). Bland-Altman plots demonstrated a narrow 95% confidence interval suggesting that wrist and mid-thigh metrics are in good agreement, except for AWK which is slightly underestimated by the mid-thigh device. Conversely, low-back accelerometers demonstrated moderate linear relationship with the wrist (r = 0.63-0.98) and the Bland-Altman results showed wide limits of agreement with significant overestimations of TST, SE, SI and underestimations of WASO, AWK, SOT. Cohens d demonstrated small differences between mid-thigh and wrist devices, except for AWK (d= 0.42). Low-back values for WASO, SE, and AWK showed moderate differences. ConclusionsThis analysis demonstrates that the mid-thigh accelerometer yields comparable sleep metrics to wrist-worn devices when processed with the van Hees algorithm.

ImportanceThe human circadian system, a critical biological mechanism governing the sleep-wake cycle, begins to oscillate before birth. Distinct fetal behavioral states have been described using short-duration ultrasound recordings, but the alignment of fetal activity with day and night cycles (i.e., diurnal rhythms) remains poorly understood due to the lack of long-term monitoring. Accordingly, a substantial knowledge gap exists concerning the evolution of fetal diurnal rhythms into infant sleep-wake cycles. ObjectiveTo investigate the development of fetal diurnal rhythms and their evolution into postnatal infant sleep, as well as the role of maternal factors and melatonin in shaping early circadian entrainment. MethodsIn this cohort study, conducted in 2022 and 2023, we collected over 20 000 hours of wearable acceleration and temperature data from 32 fetuses and their mothers during continuous 5-day monitoring in the third trimester of pregnancy. The cohort was subsequently followed longitudinally from the prenatal period through 6 months postpartum, with infant sleep outcomes assessed at three key time points (first postnatal weeks, 3 months, and 6 months) using sleep diaries and questionnaires. The first postnatal assessment additionally included breast milk and infant stool sample collection. ResultsMulti-level modelling revealed early diurnal rhythms in fetuses that align with maternal activity and circadian rhythms. Random Forest analyses identified fetal day/night sleep duration ratio - a simplified proxy for fetal circadian entrainment - as the strongest predictor of postnatal infant sleep, with fetuses preferring nighttime sleep maintaining this preference postnatally. Maternal sleep regularity (i.e., consistent sleep patterns characterized by low day-to-day variability in sleep timing) during pregnancy also predicted infant sleep, associating with a stronger preference for nighttime sleep in infants. Conclusions and relevanceThese findings highlight the potential influence of intrauterine and maternal factors on the evolution of circadian entrainment. Our study contributes to a broader understanding of the earliest emergence of circadian rhythms, supporting future research on their long-term health impacts. Maternal sleep regularity stands out as the earliest modifiable target for interventions, offering an actionable pathway to promote infant circadian entrainment, with potential long-term benefits for family dynamics and well-being.

Sex differences in sleep and wakefulness are well documented in humans but remain inconsistent in rodent studies, suggesting strong sensitivity to experimental context. In prior work, we observed no sex differences in sleep-wake architecture under relatively bright daytime light, raising the possibility that daytime illumination is a critical but underappreciated variable shaping sex-dependent sleep regulation. Here, we tested the hypothesis that daytime light intensity modulates sex differences in sleep-wake architecture and vulnerability to dim light at night (DLaN). Male and female C57BL/6J mice were exposed to acute (one night) or chronic (two weeks) DLaN (10 lux) under three daytime light intensities (50, 100, 300 lux). Sleep was assessed using electroencephalographic-based measures of vigilance states and slow wave activity (SWA). Dim daytime light (50 lux) unmasked robust sex differences in dark-phase sleep-wake architecture that were absent under brighter daytime light (300 lux). Acute DLaN reduced early-night wakefulness in both sexes under low daytime light but had minimal effect under bright daytime conditions. Following chronic DLaN, males exhibited reduced dim light-phase wakefulness and dampened rhythm amplitude, whereas females showed pronounced phase shifts, rhythm attenuation, and altered timing of SWA under 50 and 100 lux. These changes were largely prevented under bright daytime light. Together, these findings identify daytime light intensity as a critical contextual factor governing sex-specific regulation of sleep and vulnerability to nighttime light, providing a unifying framework to reconcile inconsistencies in the rodent sleep literature. HighlightsO_LIDaytime light intensity shapes sex differences in sleep-wake architecture C_LIO_LIAcute and chronic nighttime light elicit distinct sex-specific sleep responses C_LIO_LIFemales exhibit greater circadian and slow-wave vulnerability to nighttime light C_LIO_LIBrighter daytime light buffers sleep and circadian disruption C_LI

The skin is the largest human organ with a circadian clock that regulates its function. Although circadian rhythms in specific functions are known, rhythms in the proximal clock output, gene expression, in human skin have not been thoroughly explored. This work reports circadian gene expression in two skin layers, epidermis and dermis, in a cohort of young, healthy adults, who maintained natural, regular sleep schedules. 10% of the expressed genes showed rhythms at the population level, of which only a third differed between the two layers. Broadly, expression magnitudes of circadian genes were consistent across subjects in each layer. Amplitude and phases of circadian gene expression, however, varied more across subjects than layers, with amplitude being more variable than phases. Expression amplitudes in the epidermis were larger and more subject-variable, while they were smaller and more consistent in the dermis. Core clock gene expression was similar across layers at the population-level, but were heterogeneous in the their variability across subjects. We used this data to identify small sets of biomarkers for internal clock phase in each layer, which consisted of layer-specific non-core clock genes. This work provides a valuable resource to advance our understanding of human skin to realize the potential of circadian medicine as well as a novel methodology to quantify sources of variability in human circadian rhythms.

Intrinsic biological rhythms regulate key physiological and behavioural processes, yet the influence of sex and age on these rhythms is not fully understood. We comprehensively examined 24-hour (circadian) and >24-hour (infradian; 5-day and 10-day) rhythms in wheel-running and ingestive behaviours in single-housed young and middle-aged male and female mice. Circadian analysis revealed that middle-aged mice, particularly females, exhibited more precise daily rhythms and shifted a greater proportion of activity and feeding to the lights-on phase compared to young female mice. Middle-aged animals also ran for longer durations per day, suggesting age-related changes in activity regulation. Analysis of infradian rhythms further highlighted sex- and age-specific differences. Young female mice displayed robust 5-day rhythms in wheel-running activity, which were absent in middle-aged females. In contrast, few males (young or middle-aged) showed significant 5-day rhythms. Ten-day rhythms were most prominent in male mice, while females rarely expressed this periodicity. Physiologically, middle-aged mice lost more body weight in response to single housing, with middle-aged females being most affected. Interactions among behavioural rhythms in females also showed greater complexity, which increased with age. These findings reveal distinct sex- and age-dependent patterns in circadian and infradian rhythms as well as in physiological responses to isolation. Our work highlights the need to account for sex and age in chronobiological research, with broader implications for understanding vulnerability to age-related metabolic and behavioural disorders.

Circadian rhythms play a critical role in regulating physiological and behavioral processes, with amplitude being a key parameter for their characterization. However, accurately quantifying circadian amplitude in natural settings remains a challenge, as traditional melatonin methods require lab settings and are often costly and time-consuming. Wearable devices are a promising alternative as they can collect consecutive 24-h data for multiple days. The most commonly used measure of circadian amplitude from wearable device data, relative amplitude, is subject to the masking effect of behaviors and fails to leverage the rich information in high-dimensional data, as it only uses the sum of activity counts in time windows of pre-specified lengths. Therefore, in this study, we firstly proposed a pipeline to derive a novel feature to characterize circadian amplitude, named circadian activity rhythm energy (CARE), which can well address the above-mentioned challenges by decomposing raw accelerometer time series data, and then we validated the new feature CARE by assessing its correlation with melatonin amplitude (Pearsons r = 0.46, P = 0.007) in a dataset of 33 healthy participants. Secondly, we investigated its association with cognitive functions in two datasets: an adolescent dataset (Chinese SCHEDULE-A, n = 1,703) and an adult dataset (the UK Biobank dataset, n = 92,202), and we found that the CARE was significantly associated with the Global Executive Composite ({beta} = 28.02, P = 0.016) in adolescents, and reasoning ability (OR = 0.01, P < 0.001), short-term memory (OR = 3.42, P < 0.001), and prospective memory (OR = 11.47, P < 0.001) in adults. And finally, we explored the causal relationship using Mendelian randomization analysis in the adult dataset. We identified one genetic locus with 126 SNPs associated with CARE using genome-wide association study (GWAS), of which 109 variants were used as instrumental variables to conduct causal analysis. The results suggested that CARE had a significant causal effect on reasoning ability ({beta} = -59.91, P < 0.0001), short-term memory ({beta} = 7.94, P < 0.0001), and prospective memory ({beta} = 16.85, P < 0.0001). The findings suggested that CARE is an effective wearable-based metric of circadian amplitude with a strong genetic basis and clinical significance, and its adoption can facilitate future circadian studies and potential interventions to improve circadian rhythms and cognitive functions.

The Circadia Study (Circadia) is a novel "direct to participant" research study investigating the genetics of circadian rhythm disorders. The long-term goals of this study are to better understand the genetics of circadian rhythm disorders, investigate the efficacy and accessibility of an at-home, self-directed DLMO collection, to improve health outcomes in the future for patients with circadian rhythm disorders, and to address the specific needs of the circadian rhythm disorder patient population. In this manuscript, we briefly outline the standard methods of both circadian biology research protocols and circadian rhythm disorder diagnostic procedures. We describe some of the inherent limitations of current circadian research and diagnostic methods, which motivated our development of and informed the design of the Circadia Study. We discuss the main goals of the Circadia Study, and we outline key features of our study design that build upon current study methods and address limitations. Finally, we describe specific aspects of the Circadia Study, including our study population, data collection methods, and standard operating procedures so that others may replicate aspects of the study design. The Circadia Study is approved for human subject research by the Mass General Brigham Institutional Review Board, Protocol # 2020P002779.