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. The magnitude and rapidity of the shift--six hours after two exposures, Czeisler reports--suggest that the human circadian rhythm may be easier to reset than was previously believed. Therapeutic applications of bright lights may ease the frequent and sudden shift changes experienced by the estimated one-third of U.S. workers on rotating shifts.

Another experiment exploring circadian rhythms was discussed last week at meetings held at the Marine Biological Laboratory at Woods Hole, Mass. Robert Barlow placed horseshoe crab hatchlings in environments of complete light, complete dark and normal light/dark. The crabs raised in total darkness did not develop eyes, nor did they display evidence of circadian rhythms. But the all-light crabs, when placed in darkness and then exposed to a single flash of light, began to show normal rhythms. Both the all-light crabs and the light/ dark crabs had individual cycle periods of about 24 hours. This suggests, says Barlow, that the period of the biological clock is genetically set but environmental stimuli must trigger it, in a process analogous to winding up a new watch that will then run forever.