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Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle
Open accessAbstract
The electric light is one of the most important human inventions. Sleep and other daily rhythms in physiology and behavior, however, evolved in the natural light-dark cycle [1], and electrical lighting is thought to have disrupted these rhythms. Yet how much the age of electrical lighting has altered the human circadian clock is unknown. Here we show that electrical lighting and the constructed environment is associated with reduced exposure to sunlight during the day, increased light exposure after sunset, and a delayed timing of the circadian clock as compared to a summer natural 14 hr 40 min:9 hr 20 min light-dark cycle camping. Furthermore, we find that after exposure to only natural light, the internal circadian clock synchronizes to solar time such that the beginning of the internal biological night occurs at sunset and the end of the internal biological night occurs before wake time just after sunrise. In addition, we find that later chronotypes show larger circadian advances when exposed to only natural light, making the timing of their internal clocks in relation to the light-dark cycle more similar to earlier chronotypes. These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.
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“This hypothesis is supported by a recent study that found large inter-individual differences in circadian timing vanished when individuals were exposed to only natural outdoor light45.”
From Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing (2017) · cites this paper as [45]
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“Despite light being the strongest zeitgeber, this phase-advance could also result from physical exercise during daytime [102, 105], which is often confounded with time spent outdoors.”
From Effects of light on human circadian rhythms, sleep and mood (2019) · cites this paper as [102]
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“Furthermore, time spent exposed to natural sunlight helps to regulate circadian rhythms, encouraging healthy sleep-wake cycles and improved sleep [44], which is key for psychological well-being.”
From Psychological impacts of “screen time” and “green time” for children and adolescents: A systematic scoping review (2020) · cites this paper as [44]
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“Thereby, artificial light can delay the timing of the circadian clock and thus sleep [102].”
From Effects of light on human circadian rhythms, sleep and mood (2019) · cites this paper as [102]
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“For example, increasing light exposure in the natural environment shifts sleep timing earlier [91], while exposure to ambient light at night either from ereaders [92] or urban lighting [93] promotes “lateness.””
From Smoking, screen-based sedentary behavior, and diet associated with habitual sleep duration and chronotype: data from the UK Biobank (2016) · cites this paper as [91]
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Metadata
- Authors
- Wright, K. P., McHill, A. W., Birks, B. R., Griffin, B. R., Rusterholz, T., & Chinoy, E. D.
- Affiliations
- University of Colorado Boulder (US)
- Corresponding author
- Wright, Kenneth P.
- Venue
- Current Biology
- Publisher
- Elsevier BV
- Published
- 2013-08
- Volume
- 23
- Issue
- 16
- Pages
- 1554-1558
- ISSN
- 0960-9822 (ISSN-L)
- PMC
- PMC4020279
- OpenAlex
- W2031750133
- Cited by
- 783
- References
- 44
- Citations by year
- 2026: 77 · 2025: 93 · 2024: 70 · 2023: 79 · 2022: 92 · 2021: 75 · 2020: 62 · 2019: 55 · 2018: 31 · 2017: 43 · 2016: 39 · 2015: 41 · 2014: 21 · 2013: 5
- FWCI
- 18.90
- 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 Heart, Lung, and Blood Institute (R01HL081761)
- 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 clock, light-dark cycle, electric lighting, circadian entrainment, chronotype, circadian phase, sleep timing, sun exposure, artificial light at night, circadian misalignment
- MeSH terms
- Adult, Female, Humans, Lighting, Male, Sunlight, Photoperiod, Young Adult, Circadian Clocks
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