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

Feb 7, 2017Current biology : CB

Body Clock Adjustment to Natural Light and Dark Changes across Seasons and Weekends

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Abstract

Exposure to a natural light-dark cycle can shift circadian timing by approximately 69% within a weekend.

A week's exposure to a natural winter light-dark cycle leads to earlier onset of biological night and sleep compared to a modern electrical lighting environment.
The human circadian clock shows sensitivity to seasonal variations, with an expansion of biological night in winter akin to patterns observed in non-human species.
Circadian and sleep timing occur earlier after camping in a natural summer light-dark cycle than during a typical weekend in a modern environment.
Rapid adjustments in circadian timing can be achieved through short-term exposure to natural light.

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

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