Daily Rhythms Running like Clockwork
Daily rhythms running like clockwork
Bright light alone, according to one case study, can rapidly reset the human circadian rhythm--the biological clock that keeps myriad daily cycles such as sleepiness and body temperature in harmony with the environment. Though this finding is consistent with animal studies, researchers had previously thought human daily cycles were also governed by the sleep-wake cycle and by social cues.
Charles Czeisler of Brigham and Women's Hospital in Boston conducted the experiment, reported in the Aug. 8 SCIENCE. In it, a 66-year-old woman with a naturally fast-running biological clock and a regular sleep pattern sat before bright lights for four hours each night before going to bed--the time at which her unusually short cycle made her most sensitive to light. Because most people are most sensitive to light during sleep, previous studies involved waking them up--which made researchers unsure whether subsequent circadian changes resulted from light or from premature awakening.
While her sleep patterns remained the same, other aspects of the woman's circadian rhythm--such as body temperature and hormonal secretions--shifted after only one exposure. …