Stothard ER, McHill AW, Depner CM, et al. Circadian entrainment to the natural light-dark cycle across seasons and the weekend. Curr Biol. 2017;27(4):508-513.
Design and Participants
- Study 1: Five (1 woman, 4 men, aged 30.4 ± 8.6 years) healthy, physically active individuals participated in the 2-week experiment occurring around the winter solstice in Boulder, Colorado. During the first week, participants recorded regular daily sleep- and wake-times in their home environments using their typical self-selected artificial light at night (ALAN) habits. The second week was an outdoor group camping experience in the Rocky Mountains, where only natural light (ie, sunlight, moonlight, and campfires; no flashlights, lanterns, or electrical devices) was permitted.
- Study 2: Fourteen (7 men, 7 women, aged 28.4 ± 8.8 years) healthy, physically active individuals participated in a 1-week experiment occurring around the summer solstice in Boulder, Colorado. During the week, participants recorded regular daily sleep- and wake-times in their home environments using their typical self-selected ALAN habits. On the weekend, the experimental group (n=9) went for a 2-day group camping experience in the Rocky Mountains (flashlights were permitted). The remaining participants stayed in their home ALAN environments for the weekend.
- Nightly melatonin levels (including onset, midpoint, and endpoint)
- Nighttime sleep schedule (including duration, onset, and ending times)
In Study 1, winter camping participants had mean melatonin onset times occur 2.6 hours earlier than their individual baseline ALAN times (P=0.017, effect size η2G=0.57). This delay grew from baseline (ie, mean melatonin onset time was successively earlier) on each day of the 6-day camping experience, as participants adjusted to a more natural melatonin production pattern.
Melatonin onset during winter camping was significantly earlier and sleep time was significantly increased when compared with a similar summer camping experience (sleep time: 10.0 ± 1.8 hours vs 14.4 ± 2.8 hours, P<0.025, effect size η2G=0.53). In contrast, there were no significant differences in melatonin or sleep times between the winter and summer ALAN groups, demonstrating the equalizing effects of modern lighting on natural seasonal circadian rhythms.
Both of these small experiments demonstrate applications of the general philosophy of natural medicine—namely, to work in harmony with the cycles of the natural world.
In Study 2, weekend camping participants had mean melatonin onset and midpoint times occur 1.4 and 1.0 hours earlier (respectively) than their individual baseline times from the preceding week (P<0.01; effect sizes η2G=0.45 and 0.20, respectively), despite no change in sleep pattern. Participants in the ALAN baseline group had mean delays of approximately 1.0 hour for melatonin onset and midpoint, reflecting the greater social activities (eg, being with friends, watching TV/movies) reported by participants.
These studies are the first to use a controlled experimental method to demonstrate that real-world ambient light exposures (vs laboratory-induced alterations) influence melatonin onset time in humans. Study 1 definitively shows natural patterns of melatonin production can vary seasonally between winter and summer months, with corresponding changes in sleep duration, in contrast to the typical melatonin and sleep patterns resulting from ALAN exposure in modern society, which are consistent year-round.
Study 2 shows that the shifting of circadian melatonin patterns can happen quickly over a single weekend when exposed to natural vs artificial lighting patterns.
Both of these small experiments demonstrate applications of the general philosophy of natural medicine—namely, to work in harmony with the cycles of the natural world and of the vis medicatrix naturae (the healing power of nature).
Extended melatonin production and sleep times during winter months confer evolutionary advantages in terms of hormonal, neurotransmitter, and immune system regulation.1 This has clinical implications for mental health conditions (eg, seasonal affective disorder)2 and autoimmune diseases (eg, multiple sclerosis).3 Of course, circadian cycling of melatonin production is known to influence a variety of other health-related outcomes, most notably insomnia and sleep-wake cycle disorders4 and cancer, particularly breast cancer (as reported in a previous issue of Natural Medicine Journal).5 Other health conditions known to be affected by circadian disruption include adrenal insufficiency, insulin/blood sugar regulation, weight control/obesity, hypertension, cognitive functioning, Parkinson’s disease, and asthma.6
Limitations of this study include its very small sample size, lack of randomization, and self-selection of participants. Anyone who would volunteer to go camping in Colorado for a week at the end of December is probably in a unique subset of the population and may not have physiological (or psychological) responses that are representative of larger general populations. However, the results of these studies do affirm decades of circadian rhythm studies in other mammals. Further research is needed to verify the clinical benefit of this work.
Human circadian rhythms can fluctuate greatly depending on seasonal and ambient natural vs artificial light levels. While not everyone is a candidate for 6 days of winter camping in the Rocky Mountains, it is likely that everyone could benefit from adjusting their ALAN schedule to be more in harmony with the natural light and dark cycles of the seasons.