Night owls may want to dim their lights

Study finds that that night-time lighting reduces hormone associated with sleep and health

People who spend their evenings in relatively bright light run the risk of stressing their bodies by ratcheting down the production of melatonin. Produced in the brain’s pineal gland, this hormone plays a pivotal role in setting the body’s biological clock – and, potentially, in limiting the development of certain cancers.

More than 100 young adults volunteered for a roughly 10-day research trial during which each took turns living in a light-controlled room at Brigham and Women’s Hospital in Boston. From midnight until 8 a.m. the room was totally dark. At other times, researchers from Harvard Medical School tinkered with the room’s lighting.

On most evenings, the illumination averaged 200 lux (or roughly the brightness of a normal living room at night); on other evenings, it was no brighter than 3 lux (what might be expected from three candles burning at a distance of 1 meter.)  

When their room’s lighting had been bright, the participants made, on average, 71 percent less melatonin in the hours before sleep. Their bodies also commenced production of the hormone substantially later on nights when the lighting was brighter – just 23 minutes before scheduled sleep time (midnight) versus almost 2 hours before bedtime when the lighting had been dim.

What’s more, the body didn’t fully catch up for any late start on melatonin synthesis. The day’s production fell short by about 12.5 percent after an evening when the lighting was bright.

And it gets worse if people pull all-nighters, Joshua J. Gooley and his colleagues found. In a second, smaller trial involving just 12 volunteers, they let the participants spend a few days adjusting to a normal day-night lighting routine in the test room. Then came a 40-hour cycle of constant light at the 200 lux level.

Eleven people went through this ordeal once. The twelfth endured it twice. And in 11 of the 13 trials, the recruits sustained a dramatic reduction in melatonin production – of at least 51 percent; in six trials the hormone shortfall ranged from 76.9 to 92.5 percent. This experiment established that “exposure to room light in participants who were kept awake during the usual hours of sleep suppressed melatonin by more than half the amount measured during sleep in darkness,” Gooley’s team reports in the Journal of Clinical Endocrinology & Metabolism.

 Their paper, slated to appear in March, has been posted early online.

“Given that chronic light suppression of melatonin has been hypothesized to increase risk for some types of cancer and that melatonin-receptor genes have been linked to type 2 diabetes, our findings could have important health implications for shift workers who are exposed to indoor light at night over the course of many years,” Gooley says.

Richard Stevens of the University of Connecticut Health Center, in Farmington, goes farther. “I think – hope – that this paper will be seen as a turning point.” For what? For the possibility that the typical nighttime illumination to which almost all people in the modern world (not just shift workers) are exposed might actually pose a health risk, especially for breast cancer.”

Until a seminal 1980 paper in Science, “it was thought that humans, unlike other animals, were insensitive to light during the night,” Stevens says. Seven years later, when he published a paper hypothesizing that light at night might foster breast cancer, plenty of people scoffed. Indeed, he recalls, “Nobody thought room light from electric bulbs was adequate to suppress melatonin.”

The JCEM paper, he says, now suggests that indoor lighting at night not only lowers melatonin, but also alters the rhythmic cycles of the body’s clock. And that, he contends, means that light at night “could be a problem for any malady for which [those] circadian rhythms might matter — like breast cancer.”

Janet Raloff

Janet Raloff is the editor of Science News for Students, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer.

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