They have discovered what appears to be a novel "photoreceptor system" in the human eye that regulates the biological and behavioral effects of light on the body. They have elucidated the specific wavelengths of light that control the production of melatonin, which plays an important role in the body's circadian rhythms. The wavelengths of light in the blue region of the visible spectrum are the most potent in controlling melatonin production.
"This discovery will have an immediate impact on the therapeutic use of light for treating winter depression and circadian disorders," says Dr. Brainard, professor of neurology at Jefferson Medical College of Thomas Jefferson University.
"In the long range, we think this will shape all artificial lighting, whether it's used for therapeutic purposes, or for normal illumination of workplaces, hospitals or homes," he says. "Broad changes in general architectural lighting may take years, but the groundwork has been established."
He discusses his research and various other light-related topics Feb. 14 at the annual meeting of the American Association for the Advancement of Science in Denver.
Dr. Brainard, who is also associate team leader for the Human Performance Factors, Sleep and Chronobiology Team of the National Space Biomedical Research Institute in Houston, argues that the therapeutic use of light is becoming more important to society increasingly deprived of sleep. Light therapy is an already well established treatment for the winter blues, or seasonal affective disorder, a not-so-uncommon problem during long cold, wintry nights.
At the same time, Dr. Brainard and others are studying an even newer hypothesis: Over-exposure to light at night can disrupt the production of melatonin and disturb circadian rhythms in a way that raises the risk of breast cancer in women.
The hypothesis is gathering support. Breast cancer incidence is higher in modern societies, and a key feature of industrial societies includes increased electric light exposure at night.
Several scientific studies provide evidence as well, Dr. Brainard says. Researchers have shown, for example, that melatonin inhibits the growth of some cancer cells in the laboratory dish. At the same time, light-induced melatonin suppression increases tumor growth in animal studies. Epidemiological studies have found an increased breast cancer incidence in women who work night shifts, while researchers have shown a lower risk of breast cancer in blind women.
While much work remains, he says, if the hypothesis proves to be true - that exposure to light at night is a genuine risk factor for cancer development - there will need to be fundamental changes in the way societies illuminate their homes, factories and streets.
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Editors: This information is embargoed for release at a news briefing on Feb. 14 at 3 p.m. ET at the annual meeting of the American Association for the Advancement of Science in Denver. (The meeting session, Environmental Effects of Outdoor Lighting, is Feb. 15, 12:30-2 p.m. ET.)