Posted on Aug 27, 2013, 6 a.m.
A week of exposure solely to natural light synchronizes the body’s circadian rhythm to the solar day.
With the advent of electric lighting – as well as other conveniences of modern living – the human body receives fewer natural cues that otherwise signal the circadian rhythm, the body’s “internal clock,” to wakefulness and sleep. Kenneth P. Wright Jr., from the University of Colorado/Boulder (Colorado, USA), and colleagues monitored eight participants for one week as they went about their normal daily lives. The participants wore wrist monitors that recorded the intensity of light they were exposed to, the timing of that light, and their activity, which allowed the researchers to infer when they were sleeping. At the end of the week, the researchers also recorded the timing of participants’ circadian clocks in the laboratory by measuring the presence of melatonin. The same metrics were recorded during and after a second week when the eight participants went camping in the Colorado woods. During the week, the campers were exposed only to sunlight and the glow of a campfire. Flashlights and personal electronic devices were not allowed. On average, participants’ biological nighttimes started about two hours later when they were exposed to electrical lights than after a week of camping. During the week when participants went about their normal lives, they also woke up before their biological night had ended. After the camping trip—when study subjects were exposed to four times the intensity of light compared with their normal lives—participants’ biological nighttimes began near sunset and ended at sunrise. They also woke up just after their biological night had ended. Becoming in synch with sunset and sunrise happened for all individuals even though the measurements from the previous week indicated that some people were prone to staying up late and others to getting up earlier. The study authors write that: “These findings have important implications for understanding how modern light exposure patterns contribute to late sleep schedules and may disrupt sleep and circadian clocks.”
Kenneth P. Wright Jr., Andrew W. McHill, Brian R. Birks, Brandon R. Griffin, Thomas Rusterholz, Evan D. Chinoy. “Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle.” Current Biology, 1 August 2013.