Individual differences in brain clock may account for differences in preferred sleep timing; Modulated light exposure could treat circadian rhythm sleep disorders associated with space flight, shift work, and more.
The following news release was prepared by Brigham and Women’s Hospital.
BOSTON-Researchers from Brigham and Women’s Hospital’s (BWH) Division of Sleep Medicine and colleagues, have found that by giving individuals two, 45-minute exposures to bright light pulses in the evening they could entrain (synchronize) a person’s circadian system to function properly in days longer than the usual 24-hour light/dark cycle. The study was conducted for NASA’s National Space Biomedical Research Institute, and the findings can be applied to the planned year-and-a-half visit to Mars, where the Martian day is 24.65 hours long. Without the ability to reset the internal clock to endure the longer day, an individual would feel as if they were in a perpetual state of jet lag. The findings appear in the May 15, 2007 issue of the Proceedings of the National Academy of Sciences.
The study involved 12 healthy participants between 22-to-33 years of age who had maintained a regular 8-hour sleep and 16-hour wakeful schedule at home for at least three weeks before the start of the study. Participants stayed in individual rooms free from external time cues for a 65-day span with scheduled exposure to light, opportunities to sleep, eat and take showers based on a standard 24-hour pattern.
As a first step, the researchers determined the intrinsic period of each subject’s inner brain clock. They found that individual differences in this fundamental property of the internal brain clock determined the timing of the release from the pineal gland of the sleep-promoting hormone melatonin, which has implications for understanding biological differences in the timing of preferred sleep times (for example, morning and evening types). Next, the participants began a 30-day period of longer-than-24-hour days in one of three light/dark conditions including using modulated light exposure. Scenario number one was exposure to dim light (25 lux), room lighting (100 lux) and a modulated light exposure of dim light (25 lux) for the first 10 hours of the waking day, followed by room light of 100 lux for the remainder of the waking day and two, 45-minute exposures to bright light pulses of approximately 9500 lux (equivalent to full daylight).
The researchers found that participants exposed to the modulated light exposure, which included the two evening bright light pulses, were able to synchronize their sleep/wake cycles to a longer than 24-hour day. Dr. Charles Czeisler, chief of the Division of Sleep Medicine at Brigham and Women’s Hospital, Baldino Professor of Sleep Medicine at Harvard School of Medicine and Senior Author of the study commented, "The results have powerful implications for the treatment of circadian rhythm sleep disorders, including shift work disorder and advanced sleep phase disorder. With human long-duration missions planned to the Moon and Mars, the results of our research should enable astronauts to synchronize their circadian pacemaker, or internal clock, to the 24.65-hour long day found on Mars. Without being able to synchronize their internal circadian clock they would be in a constant state of jet lag, which could lead to sharp decreases in the level of performance, jeopardizing the health and safety of the crews."
The research was supported by grants from the National Aeronautics and Space Administration Cooperative Agreement NCC 9-58 with the National Space Biomedical Research Institute, NASA grant NAG 5-3952 and the National Center for Research Resources – National Institutes of Health Grant.
Dr. Czeisler’s co-authors included Drs. Claude Gronfier; Universite Lyon, France; Kenneth P. Wright, Jr. University of Colorado, Boulder and Richard E. Kronauer, Harvard University, Cambridge, MA.