PITTSBURGH (AP) _ After three months inspace, astronauts lose sleep and sleep less soundly because the lack of gravityand 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 astronautsless alert and hamper performance on longer trips, according to the study,published in the December issue of the journal Psychosomatic Medicine.

Space sleeplessness is apparentlycaused by changes in the brain's endogenous circadian pacemaker _ a bundle ofnerve cells the size of a pinhead that controls the body's cycle of sleep andwakefulness.

Among the influences on the internalclock, researchers believe, is light hitting the retina of the eye to help thebody determine what time of day it is. Researchers also said the lack ofgravity in space means astronauts expend much less energy than on Earth and donot tire as easily.

``Our study shows that humankind mayneed to find ways to trick the (internal clock) into maintaining a strong24-hour cycle if we are to succeed on longer missions,'' said Timothy Monk, theUniversity of Pittsburgh Medical Center psychiatry professor who led the study.

For the study, which was funded byNASA, Pittsburgh researchers monitored the sleep patterns of retired astronautJerry Linenger during nearly five months aboard the Russian space station Mirin 1997.

During three two-week periods in thebeginning, middle and end of Linenger's trip, researchers on the ground loggedhis sleep patterns _ when he went to bed, how many times he woke up and howlong he slept. Meanwhile, Linenger recorded his temperature, kept a sleep log,rated his alertness five times a day and took tests at midday.

In the beginning of his Mir stay, hefollowed a regular bedtime and rising routine, but after four months of dayswith 15 sunrises and sunsets, ``I lost my sense of day and night,'' Linengersaid.

``Every 45 minutes: light dark, lightdark. So biorhythms are totally out of whack,'' Linenger said, adding he beganwaking throughout his sleep period.

Others aboard suffered similar problems,Linenger said.

``You'd be talking to them middle ofthe day, they'd just fall asleep in the middle of the conversation,'' he said.

Although there were a fire, coolantleaks and a near collision during his stay, both Linenger and Monk said thosethings did not significantly affect his sleep.

Past studies have found that others whoare shut off from normal light-dark cycles, including blind people, sufferinsomnia and daytime sleepiness or have a sleep-wake pattern that extends morethan a day.

Researchers have tried a variety ofremedies for sleeplessness in space, including sleeping pills; melatonin, ahormone put out by the pineal gland that is thought to control the sleep cycle;and using bright lights to mimic sunlight.

The Pittsburgh researchers alsosuggested modifying how light is used in the spacecraft and using exercise tocounteract the effects of weightlessness.