In a world lit bright by artificial light, the study has obvious implications for what drives our sleep cycles.
"The average American spends less than a half an hour in light exceeding 1,000 lux-even in places like sunny San Diego, let alone places like Boston," says Charles Czeisler, HMS associate professor of medicine, whose laboratory conducted the research. "Given this limited exposure to natural light, indoor light becomes the primary source for resetting our biological clocks."
Czeisler and his colleagues reported their findings in the Feb. 6 issue of Nature. Diane Boivin, research fellow in medicine, was lead author of the study.
To investigate the clock-setting power of artificial light, the researchers divided 31 male subjects into four groups. The groups were put on an inverted sleep-wake cycle for three days. They were kept awake at night and slept during the day.
For the first five hours of their waking "day," the subjects were kept in very dim light. Three groups were then exposed to a different intensity of light, ranging from ordinary room light to bright light to very bright light. The control group remained in dim light for the three days of the experiment.
Body temperature-which typically reaches its lowest point two hours before a person habitually wakes-was monitored continuously at one-minute intervals. The body- temperature nadir was used as a marker of the circadian rhythm.
The one hour "advance" triggered by ordinary room light is exactly the amount predicted by Richard Kronauer, Gordon McKay Professor of Mechanical Engineering. In 1987, he devised a mathematical model predicting that even very low intensity light would have a significant resetting effect. The results predicted by Kronauer's model closely match the observed results Czeisler and his colleagues report in the Feb. 26 issue of the American Journal of Physiology.
"We were pleased with the concordance between the model and the experimental results," says Elizabeth Klerman, instructor in medicine and lead author of the article in the American Journal of Physiology.
In modern society, of course, we are regularly exposed to artificial light, both in the work place and after sunset. Czeisler believes that this extended exposure to artificial light late into the night, along with the shielding from sunlight by curtains and shades early in the morning, could be wreaking havoc with our natural biological clocks. "Together the two things shift the circadian system to a later time probably by a substantial number of hours," he says.
In other words, our constant exposure to artificial light is leaving our bodies out of sync with the light rhythms of the natural world. One way to get the body's natural rhythms back in sync is to rough it, Czeisler said. "People who camp report they sleep very well even though they are often in poor sleeping conditions."
--Misia Landau
