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Circadian Entrainment to the Natural Light-Dark Cycle across Seasons and the Weekend
Open accessAbstract
Reduced exposure to daytime sunlight and increased exposure to electrical lighting at night leads to late circadian and sleep timing [1-3]. We have previously shown that exposure to a natural summer 14 hr 40 min:9 hr 20 min light-dark cycle entrains the human circadian clock to solar time, such that the internal biological night begins near sunset and ends near sunrise [1]. Here we show that the beginning of the biological night and sleep occur earlier after a week's exposure to a natural winter 9 hr 20 min:14 hr 40 min light-dark cycle as compared to the modern electrical lighting environment. Further, we find that the human circadian clock is sensitive to seasonal changes in the natural light-dark cycle, showing an expansion of the biological night in winter compared to summer, akin to that seen in non-humans [4-8]. We also show that circadian and sleep timing occur earlier after spending a weekend camping in a summer 14 hr 39 min:9 hr 21 min natural light-dark cycle compared to a typical weekend in the modern environment. Weekend exposure to natural light was sufficient to achieve ∼69% of the shift in circadian timing we previously reported after a week's exposure to natural light [1]. These findings provide evidence that the human circadian clock adapts to seasonal changes in the natural light-dark cycle and is timed later in the modern environment in both winter and summer. Further, we demonstrate that earlier circadian timing can be rapidly achieved through natural light exposure during a weekend spent camping.
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“Here, shorter daylight exposure and longer nights are associated with a longer biological night as indexed by the duration of melatonin secretion, and thus longer sleep duration [85, 94, 95], which may also reflect a seasonality effect [104].”
From Effects of light on human circadian rhythms, sleep and mood (2019) · cites this paper as [85]
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“Natural daylight at high intensities as experienced outside buildings has previously been shown to (1) advance the timing of sleep to earlier hours, (2) affect the duration of sleep, and (3) improve sleep quality.”
From Effects of light on human circadian rhythms, sleep and mood (2019) · cites this paper as [85]
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“Recent experimental work demonstrates that the human circadian system remains responsive to seasonal variation in the natural light-dark cycle despite widespread exposure to artificial lighting.”
From Seasonal Variation in Sleep, Respiratory, and Cardiovascular Parameters Assessed by a Wearable Device (2026) · cites this paper as Stothard et al., 2017
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“Stothard et al. (2017) showed that the timing and duration of the biological night shift in response to seasonal photoperiod, with an expansion of biological night during winter and earlier circadian timing following exposure to natural light conditions.”
From Seasonal Variation in Sleep, Respiratory, and Cardiovascular Parameters Assessed by a Wearable Device (2026) · cites this paper as Stothard et al., 2017
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“Importantly, even short-term exposure to natural light (e.g., a weekend of camping) was sufficient to induce substantial shifts in circadian phase, indicating that the human circadian clock retains sensitivity to seasonal photic cues in modern environments.”
From Seasonal Variation in Sleep, Respiratory, and Cardiovascular Parameters Assessed by a Wearable Device (2026) · cites this paper as Stothard et al., 2017
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Metadata
- Authors
- Stothard, E. R., McHill, A. W., Depner, C. M., Birks, B. R., Moehlman, T. M., Ritchie, H. K., Guzzetti, J. R., Chinoy, E. D., LeBourgeois, M. K., Axelsson, J., & Wright, K. P.
- Affiliations
- University of Colorado Boulder (US)
- Stockholm University (SE)
- Corresponding author
- Wright, Kenneth P.
- ORCID
- Venue
- Current Biology
- Publisher
- Elsevier BV
- Published
- 2017-02
- Volume
- 27
- Issue
- 4
- Pages
- 508-513
- ISSN
- 0960-9822 (ISSN-L)
- PMC
- PMC5335920
- OpenAlex
- W2585958835
- Cited by
- 303
- References
- 40
- Citations by year
- 2026: 21 · 2025: 37 · 2024: 25 · 2023: 35 · 2022: 36 · 2021: 52 · 2020: 25 · 2019: 41 · 2018: 23 · 2017: 7
- FWCI
- 16.06
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Licence
- Locations
- Current Biology (publishedVersion)
- PubMed (publishedVersion)
- Europe PMC (PubMed Central) (acceptedVersion)
- PubMed Central (acceptedVersion)
- Funders
- National Institutes of Health
- National Heart, Lung, and Blood Institute (R01HL109706)
- National Institute of Diabetes and Digestive and Kidney Diseases (R21DK092624)
- Topics
- Life Sciences › Neuroscience › Endocrine and Autonomic Systems › Circadian rhythm and melatonin
- Social Sciences › Psychology › Experimental and Cognitive Psychology › Sleep and Work-Related Fatigue
- Social Sciences › Psychology › Experimental and Cognitive Psychology › Sleep and related disorders
- Keywords
- circadian entrainment, seasonal variation, sleep timing, circadian timing, electric lighting, sun exposure
- MeSH terms
- Adult, Circadian Rhythm, Female, Humans, Lighting, Male, Seasons, Sunlight, Time Factors, Young Adult, Circadian Clocks
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