Astronaut Study Finds Sleep Loss

After three months in space, astronauts sleep less soundly and lose sleep because the lack of gravity and the absence of day-and-night cues throw off their internal clocks, according to the first long-term study of the topic. The phenomenon could leave astronauts less alert and hamper performance on longer trips, according to a study in the December edition of Psychosomatic Medicine.

Space sleeplessness is apparently caused by changes in the brain's circadian pacemaker--a bundle of nerve cells the size of a pinhead that controls the body's cycle of sleep and wakefulness. Among the influences on the internal clock, researchers believe, is light hitting the retina of the eye to help the body determine the time of day.

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Compiled by Times medical writer Thomas H. Maugh II