Fourteen physically active individuals (seven female) aged 28.4 ± 8.8 years (mean ± SD) completed a 7-day protocol ( Figures S1 B and S1C) during July in Colorado, USA, at latitude ∼40°N and longitude between 105°W and 106°W. The melatonin rhythm was first assessed in the laboratory after 2 days of maintaining habitual self-selected sleep times in the modern electrical lighting environment. Afterward, participants spent the weekend either in the modern environment (weekend electrical lighting group, n = 5) or in the natural environment backcountry camping in the Rocky Mountains of Colorado, USA, and slept in tents (weekend natural light group, n = 9; self-selected). Unlike in our prior study [] and in study 1 above, participants in the weekend natural light group were permitted to use flashlights or headlamps.

Participants in the weekend camping group were exposed to illuminance levels that were more than four times higher during waking hours while weekend camping (9,181 ± 2,229 lux) than while in the modern electrical lighting environment before camping (2,070 ± 1,120 lux; p < 0.0001; Figure S4 ). This >4-fold increase in light exposure during weekend summer camping is similar to that previously reported [] but smaller than the >13-fold increase seen during winter camping in study 1. The latter appears to be largely driven by less light exposure in the modern electrical lighting environment during winter versus summer (study 1 versus study 2; p < 0.05), consistent with [], and not by a difference in light exposure during the natural winter and weekend summer light-dark cycle (study 1 versus study 2; p = 0.36). Additionally, time spent exposed to illuminance levels above 50, 100, 550, and 1,000 lux and average exposures to red, green, and blue wavelengths of light during the waking day were significantly higher during natural light exposure in the weekend camping group than while in the modern electrical lighting environment before camping and compared to the weekend modern electrical lighting group ( Figure S4 Table S2 ).

Average timing of melatonin onset (black upward triangles), midpoint (red squares), and offset (blue downward triangles) after 2 weekdays exposure to electrical and natural lighting in the modern environment and following 2 weekend days exposure to electrical and natural light in the modern environment or 2 weekend days exposure to natural light while camping. Average sunrise and sunset times for the study are denoted by vertical black lines. Average sleep start and end times are indicated by the black bar for each weekday and weekend part of the study. Clock time denotes local time. Data are represented as mean ± SD. See also Figures S1–S4 and Table S2

1 Wright Jr., K.P.

McHill A.W.

Birks B.R.

Griffin B.R.

Rusterholz T.

Chinoy E.D. Entrainment of the human circadian clock to the natural light-dark cycle.

38 Wehr T.A.

Giesen H.A.

Moul D.E.

Turner E.H.

Schwartz P.J. Suppression of men’s responses to seasonal changes in day length by modern artificial lighting.

23 Crowley S.J.

Carskadon M.A. Modifications to weekend recovery sleep delay circadian phase in older adolescents.

39 Wittmann M.

Dinich J.

Merrow M.

Roenneberg T. Social jetlag: misalignment of biological and social time.

16 Roenneberg T.

Allebrandt K.V.

Merrow M.

Vetter C. Social jetlag and obesity.

40 Lewy A.J.

Lefler B.J.

Emens J.S.

Bauer V.K. The circadian basis of winter depression.

In summary, our findings demonstrate that the human melatonin rhythm adapts to short summer and long winter nights when living in a natural light-dark cycle—something that has been assumed but never demonstrated with respect to the “natural light-dark cycle.” We further show that living in the modern electrical lighting environment reduces seasonal circadian responsiveness by delaying the beginning of the biological night in both winter and summer []. It has been argued that humans live in a constant summer photoperiod (i.e., duration of light exposure) in the modern electrical environment [], and this appears to be true for the duration of biological night but not for circadian timing. Our finding of later circadian timing in the modern environment across seasonal extremes at the latitude studied indicates that modern lighting environmental conditions do not entirely replicate living in the natural summer photoperiod. Relatedly, we observed that the timing of the middle of the biological night, but not the middle of the sleep episode, occurs close to the timing of the middle of solar darkness when living in natural winter and summer light-dark cycles, but less so when living in the modern environment ( Figure 4 ). Our findings also show that a weekend of camping prevents the typical weekend circadian and sleep delay [], which is an important contributor to the phenomenon of social jet lag. Specifically, the weekend phase delay in the modern electrical lighting environment contributes to social jet lag on Monday morning because there is a mismatch between biological (circadian delay) and social (awakening early for work/school) timing, the definition of social jet lag []. This suggests that weekend exposure to the natural light-dark cycle may help with social jet lag [], and also with initiating treatment for winter depression [] and circadian rhythm sleep-wake disorders (e.g., delayed sleep-wake phase) that show late sleep and/or circadian timing; additional research is needed in such populations. The current studies were conducted on a relatively small number of healthy, physically active individuals; therefore, additional studies are needed with a greater number of participants at different latitudes and of different cultures to increase our understanding of how geographical location, activity, prior light history, and other variables may impact human circadian physiology in response to the natural light-dark cycle. Because we did not randomize subjects to group in study 2, or to condition order in either study, randomized, crossover studies are needed for replication and to assess how long it takes for the effects of the natural light environment on the timing of the circadian clock to be reversed by the modern electrical lighting environment. Further, additional research is also needed to determine how quickly the human circadian clock adapts to natural winter light-dark cycles and to seasonal changes further away from the equator, as well as the potential consequences of seasonal circadian responses for humans. Lastly, studies are needed to test potential health benefits of increased exposure to natural light through effects on the circadian timekeeping system.