Circadian clock out of whack? Go sleep under the stars!

TLDR: A week away from artificial light shifts back your out-of-whack circadian rhythm so that you wake at dawn and sleep at dusk. 

Artificial lighting is, for lack of a better word, a godsend. Unfortunately, our circadian rhythms don’t quite agree. Evolved over several millennia, circadian clocks are exquisitely entrained to natural light-dark cycles and allow our bodies to run “on optimal time”. During the bright-lit day, our internal clock tells us to increase energy intake and expenditure and cognition; after sundown, it promotes sleep by increasing the levels of melatonin. Since a major driving force of the circadian clock is light, how much can artificial electrical lighting drive it out of whack (I’m looking at you, iPad)? And is there any way to get our internal circadian rhythm back?

Kenneth P Wright Jr et al (2013) Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle. Current Biology. Doi: 10.1016/j.cub.2013.06.039

Picture this. In the busy streets of Colorado, 8 people, aged 20-30 years old, are going about their busy lives (and sleep) as usual. What’s out of the norm is that they are participating in an experiment: all of them wore wrist activity monitors to measure their average activity levels, sleep start time, wake time and sleep duration and efficiency. A week later, researchers took these participants into a dim-lit lab, and measured their “natural” circadian rhythms by quantifying the levels of melatonin in their saliva (which, according to the authors is the most accurate way to track circadian clocks). Participants were also asked about their usual sleeping habits.

The group then ditched the city for the Rocky Mountains, where only natural light was permitted. So no flashlights, no iPhones/Androids, only sunlight and campfires. Participants still slept as they pleased, and went back into the lab for a second measuring a week later.

And the results? During the week of camping out, participants experienced more than 400% the amount of total light than in the city, and this increase was seen throughout the day at several light intensities – expect in the evening between sunset and sleep, where participants were exposed to more (artificial) light normally. This bump in light exposure before bed is bad news as the human circadian clock is most sensitive to light-induced delays in the evening. In other words, the circadian clock thinks it’s earlier in the day than it actually is, and holds off on producing melatonin to promote sleep.

This is indeed the case. As you can see in the graph above, when engaged in their normal schedules (“Electrical Lighting”), the participants’ melatonin levels rose long after sunset, about 2 hrs before bedtime (12:30am), and remained high even AFTER wake time at ~8:00am. Since a drop in the hormone's levels contributes to wakefulness, this could explain the “morning drowsiness” that many night-owls feel after crawling out of bed.

After a week of natural light, things seemed to “reset”. As you can see in the part of the graph labelled “Natural Light”, the participants' circadian clocks shifted ~2hrs earlier on average, so that now melatonin levels rise near sunset and lower at sunrise. This correlated with a shift towards earlier sleeping and waking times, so that the participants “naturally” awoke at dawn and fell asleep at dusk; however, total sleep time did not change.

Perhaps it’s not surprising that a week away from artificial lighting can put you back “in tune” with your natural circadian rhythm. Nevertheless, this study does raise some questions. Would artificial lighting also influence circadian rhythms of people living near the equator, where there’s naturally much more sun exposure? What about people living in the arctic or Antarctic? This study was done in healthy people - would a similar effect occur in people with sleeping disorders, like insomnia? Can avoiding artificial light – on its own or with therapy – reset their circadian clocks and help them sleep?

A wonky circadian clock is linked to MANY diseases, such as sleep disorders, obesity, diabetes and heart problems. Maybe the best way to enjoy the rest of summer is to go out in the woods, sleep under the stars, and get your body back on the right “time” track.

Kenneth Wright Jr. (2013). Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle Current Biology DOI: 10.1016/j.cub.2013.06.039