After three months in space, astronauts lose sleep and sleep less soundly 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. Space sleeplessness is apparently caused by changes in the brain’s endogenous circadian pacemaker _ a bundle of nerve cells the size of a pinhead that controls the body’s cycle of sleep and wakefulness.
