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Curr Biol. 2017 Feb 20;27(4):508-513. doi: 10.1016/j.cub.2016.12.041. Epub 2017 Feb 2.

Circadian Entrainment to the Natural Light-Dark Cycle across Seasons and the Weekend.

Author information

1
Department of Integrative Physiology, Sleep and Chronobiology Laboratory, University of Colorado Boulder, Boulder, CO 80309-0354, USA.
2
Sleep and Development Laboratory, 1725 Pleasant Street, Clare Small 114, University of Colorado Boulder, Boulder, CO 80309-0354, USA.
3
Department of Clinical Neuroscience, Karolinska Institutet, K8, Psychology Axelsson, Nobels Väg 9, 17177, Stockholm, Sweden; Stress Research Institute, Stockholm University, 14419 Stockholm, Sweden.
4
Department of Integrative Physiology, Sleep and Chronobiology Laboratory, University of Colorado Boulder, Boulder, CO 80309-0354, USA. Electronic address: kenneth.wright@colorado.edu.

Abstract

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.

KEYWORDS:

advance; circadian phase; delay; non-visual; photoperiod; physiology; scotophase; spectral; spectrum; wavelength

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