Seasonal Variation in the Responsiveness of the Melanopsin System to Evening Light: Why We Should Report Season When Collecting Data in Human Sleep and Circadian Studies

Characteristics of objectively-measured naturalistic light exposure patterns in U.S. adults: A cross-sectional analysis of two cohorts

Light is an environmental feature important for human physiology. Investigation of how light affects population health requires exposure assessment and personal biomonitoring efforts. Here, we derived measures of amount, duration, regularity, and timing from objective personal light (lux) measurement in >4000 participants across two United States (US)-based cohort studies, the Multi-Ethnic Study of Atherosclerosis (MESA) and the Hispanic Community Health Study / Study of Latinos (HCHS/SOL), encompassing eight geographic regions. Objective light and actigraphy data were collected over a week using wrist-worn devices (Actiwatch Spectrum). Cohort-stratified light exposure metrics were analyzed in relation to sex, season, time-of-day, location, and demographic and sleep health characteristics using Spearman correlation and linear and logistic regressions (separately by cohort) adjusted for age, sex (where applicable), and exam site. Light exposure showed sex-specific patterns and had seasonal, diurnal, geographic, and demographic and sleep health-related correlates. Results between independent cohorts were strongly consistent, supporting the utility and feasibility of light biomonitoring. These findings provide a fundamental first characterization of light exposure patterns in a large US sample and will inform future work to incorporate light as a biologically relevant exposure in environmental public health and key component of the human exposome.

Light Environment of Arctic Solstices is Coupled With Melatonin Phase-Amplitude Changes and Decline of Metabolic Health

Light environment in the Arctic differs widely with the seasons. Studies of relationships between objectively measured circadian phase and amplitude of light exposure and melatonin in community-dwelling Arctic residents are lacking. This investigation combines cross-sectional (n = 24-62) and longitudinal (n = 13-27) data from week-long actigraphy (with light sensor), 24-h salivary melatonin profiles, and proxies of metabolic health. Data were collected within the same week bracketing spring equinox (SE), and winter/summer solstices (WS/SS). Drastic seasonal differences in blue light exposure (BLE) corresponded to seasonal changes in the 24-h pattern of melatonin, which was phase delayed and reduced in normalized amplitude (NA) during WS/SS compared to SE. The extent of individual melatonin's acrophase and Dim Light Melatonin Onset (DLMO) change from SE to WS correlated with that from SE to SS. Although similar in extent and direction, melatonin phase changes versus SE were linked to morning BLE deficit in WS, contrasting to evening BLE excess in SS. Seasonal changes in sleep characteristics were closely associated with changes in the phases of BLE and melatonin. Proxies of metabolic health included triglycerides (TG), high-density lipoprotein cholesterol (HDL), TG/HDL ratio, and cortisol. Adverse seasonal changes in these proxies were associated with delayed acrophases of BLE and melatonin during WS and SS. TG and TG/HDL were higher in WS and SS than in SE, and cross-sectionally correlated with later melatonin and BLE acrophases, while lower HDL was associated with later BLE onset and later melatonin acrophase. Overall, this study shows that optimal 24-h patterns of light exposure during SE is associated with an earlier acrophase and a larger 24-h amplitude of melatonin, and that both features are linked to better metabolic health. Improving light hygiene, in particular correcting winter morning light deficit and summer evening light excess may help maintain metabolic health at high latitudes. Novel solutions for introducing proper circadian light hygiene such as human-centric light technologies should be investigated to address these issues in future studies.

Relating Photoperiod and Outdoor Temperature With Sleep Architecture in Patients With Neuropsychiatric Sleep Disorders

While artificial light in urban environments was previously thought to override seasonality in humans, recent studies have challenged this assumption. We aimed to explore the relationship between seasonally varying environmental factors and changes in sleep architecture in patients with neuropsychiatric sleep disorders by comparing two consecutive years. In 770 patients, three-night polysomnography was performed at the Clinic for Sleep & Chronomedicine (St. Hedwig Hospital, Berlin, Germany) in 2018/2019. Sleep times were adjusted to patients' preferred schedules, patients slept in, and were unaware of day-night indicators. Digital devices and clocks were not allowed. Days were spent outside the lab with work or naps not allowed. After exclusions (mostly due to psychotropic medication), analysis was conducted on the second PSG-night in 377 patients (49.1 ± 16.8 year; 54% female). Sleep parameters were plotted as 90-day moving-averages (MvA) across date-of-record. Periodicity and seasonal windows in the MvA were identified by utilizing autocorrelations. Linear mixed-effect models were applied to seasonal windows. Sleep parameters were correlated with same-day photoperiod, temperature, and sunshine duration. The MvA of total sleep time (TST) and REM sleep began a 5-month-long decline shortly after the last occurrence of freezing 24-h mean temperatures (correlation of TST between 2018 and 2019 at 2-month lag: rs361 = 0.87, p < 0.001; maximum peak-to-nadir amplitude: ΔTST ~ 62 min, ΔREM ~ 24 min). The MvA nadirs of slow wave sleep (SWS) occurred approximately at the autumnal equinox (correlation between 2018 and 2019: rs361 = 0.83, p < 0.001). Post hoc testing following significant linear mixed-effect model indicate that TST and REM sleep were longer around November till February than May till August (ΔTST = 36 min; ΔREM = 14 min), while SWS was 23 min longer around February till May than August till November. Proportional seasonal variation of SWS and of REM sleep as percentages of TST differed profoundly (SWS = 31.6%; REM = 8.4%). In patients with neuropsychiatric sleep disorders living in an urban environment, data collected in 2018 show similar patterns and magnitudes in seasonal variation of sleep architecture as the 2019 data. Interestingly, whereas SWS patterns were consistent between years with possible links to photoperiod, annual variations of TST and REM sleep seem to be related to times of outside freezing temperature. For generalization, the data need to be confirmed in a healthy population. No clinical trial was registered.

Effects of light on biological functions and human sleep

The nonvisual effects of light in humans are mainly conveyed by a subset of retinal ganglion cells that contain the pigment melanopsin which renders them intrinsically photosensitive (= intrinsically photosensitive retinal ganglion cells, ipRGCs). They have direct connections to the main circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus and modulate a variety of physiological processes, pineal melatonin secretion, autonomic functions, cognitive processes such as attention, and behavior, including sleep and wakefulness. This is because efferent projections from the SCN reach other hypothalamic nuclei, the pineal gland, thalamus, basal forebrain, and the brainstem. The ipRGCs also directly impact the prefrontal cortex and the perihabenular nucleus (mood). In particular, light suppresses the secretion of melatonin in a dose-dependent manner, mainly depending on irradiance and spectral composition of light. There is evidence that exposure to light-emitting devices from luminaires and screens before bedtime can impact on sleep onset latency, sleep duration, and sleep quality. Likewise, light exposure during daytime modulates sleep architecture, duration, and sleep quality during the subsequent night. Therefore, the integration of acute, circadian, and long-term effects of light together influence sleep-wake quality and behavior in healthy individuals, as well as in patients with psychiatric or medical disorders.

Towards an evidence-based integrative lighting score: a proposed multi-level approach

Background: Human circadian clocks are synchronized daily with the external light-dark cycle and entrained to the 24-hour day. There is increasing evidence that a lack of synchronization and circadian entrainment can lead to adverse health effects. Beyond vision, light plays a critical role in modulating many so-called non-visual functions, including sleep-wake cycles, alertness, mood and endocrine functions. To assess (and potentially optimize) the impact of light on non-visual functions, it is necessary to know the exact 'dose' (i.e. spectral irradiance and exposure duration at eye level) of 24-hour light exposures, but also to include metadata about the lighting environment, individual needs and resources.

Problem statement: To address this problem, a new assessment tool is needed that uses existing metrics to provide metadata and information about light quality and quantity from all sources. In this commentary, we discuss the need to develop an evidence-based integrative lighting score that is tailored to specific audiences and lighting environments. We will summarize the most compelling evidence from the literature and outline a future plan for developing such a lighting score using internationally accepted metrics, stakeholder and user feedback.

Conclusion: We propose a weighting system that combines light qualities with physiological and behavioral effects, and the use of mathematical modelling for an output score. Such a scoring system will facilitate a holistic assessment of a lighting environment, integrating all available light sources.

Acute effects of light during daytime on central aspects of attention and affect: A systematic review

Light regulates both image- and various non-image forming responses in humans, including acute effects on attention and affect. To advance the understanding of light's immediate effects, this systematic review describes the acute effects of monochromatic/narrow bandwidth and polychromatic white light during daytime on distinct aspects of attention (alertness, sustained attention, working memory, attentional control and flexibility), and measures of affect (self-report measures, performance-based tests, psychophysiological measures) in healthy, adult human subjects. Original, peer-reviewed (quasi-) experimental studies published between 2000 and May 2024 were included according to predefined inclusion and exclusion criteria. Study quality was assessed, and results were synthesized across aspects of attention and affect and grouped according to light interventions; monochromatic/narrowband-width or polychromatic white light (regular white, bright white, and white with high correlated color temperature (CCT)). Results from included studies (n = 62) showed that alertness and working memory were most affected by light. Electroencephalographic markers of alertness improved the most with exposure to narrow bandwidth long-wavelength light, regular white, and white light with high CCT. Self-reported alertness and measures of working memory improved the most with bright white light. Results from studies testing the acute effects on sustained attention and attentional control and flexibility were inconclusive. Performance-based and psychophysiological measures of affect were only influenced by narrow bandwidth long-wavelength light. Polychromatic white light exerted mixed effects on self-reported affect. Studies were strongly heterogeneous in terms of light stimuli characteristics and reporting of light stimuli and control of variables influencing light's acute effects.

The effects of daylight duration on the multiple sleep latency test (MSLT) results: A pilot study

OBJECTIVE

MSLT results are known to be affected by multiple factors including sleep time, frequency of nighttime arousals, and medications intake. Although being the main synchronizer of sleep and wakefulness, daylight duration effects on MSLT have not been examined. Burlington, Vermont, USA experiences great variations in daylight duration, ranging from 8 h 50 min to 15 h 33 min of daylight. The aim of this study was to test the hypothesis that there would be photoperiod duration effects on MSLTs performed during short daylight (short daylight studies, SDS) vs. long daylight (long daylight studies, LDS) from 2013 to 2023 in our sleep laboratory.

METHODS

We identified and analyzed 37 SDS (daylight 530-560 min) and 36 LDS (daylight 903-933 min) from our database. Groups of SDS and LDS results were compared using non-paired student T test, Chi-Square and non-parametric Mann Whitney U Test.

RESULTS

Average daylight duration was 15 h 18 ± 14.6 min for LDS and 8 h 57 ± 18 min for SDS. Two groups did not differ in terms of the age, gender, BMI and race of patients studied. Mean total sleep time and sleep efficiency during PSG preceding MSLT, and MSLT mean sleep onset latency did not significantly differ for the two groups. However, SDS MSLT naps had significantly more sleep onset REM periods (SOREMP), and distribution of the number of SOREMP captured during MSLT was different for SDS and LDS groups. Differences of SDS and LDS results did not relate to sleep architecture of the overnight PSG as analysis of sleep and REM latency and relative percentages of N1, N2, REM, and N3 was not significantly different between SDS and LDS. The two groups showed difference in arousal indexes during N1 and REM sleep.

CONCLUSIONS

Daylight duration may impact MSLT results and should probably be accounted for in MSLT interpretation. Attention to photoperiod could be considered in MSLT guidelines, if our results are replicated in larger samples.

Primer on Reproducible Research in R: Enhancing Transparency and Scientific Rigor

Achieving research reproducibility is a precarious aspect of scientific practice. However, many studies across disciplines fail to be fully reproduced due to inadequate dissemination methods. Traditional publication practices often fail to provide a comprehensive description of the research context and procedures, hindering reproducibility. To address these challenges, this article presents a tutorial on reproducible research using the R programming language. The tutorial aims to equip researchers, including those with limited coding knowledge, with the necessary skills to enhance reproducibility in their work. It covers three essential components: version control using Git, dynamic document creation using rmarkdown, and managing R package dependencies with renv. The tutorial also provides insights into sharing reproducible research and offers specific considerations for the field of sleep and chronobiology research. By following the tutorial, researchers can adopt practices that enhance the transparency, rigor, and replicability of their work, contributing to a culture of reproducible research and advancing scientific knowledge.