Astronauts' sleep may get lost in space. (Science News of the week).

During a 5-month stint on the space station Mir in 1997, U.S. astronaut Jerry M. Linenger and his two Russian counterparts confronted a severe fire, failures of the oxygen generator and communications systems, and a near-collision with a resupply ship. As if that weren't enough, Linenger also found himself wrestling with his own biology: After 3 months in space, his body's clock apparently lost its daily rhythm.

One of the worst aspects of this circadian setback was that it severely disrupted Linenger's sleep, according to a new report. The scientists suspect that something functioned differently in Linenger's brain area that regulates cycles of sleep, wakefulness, alertness, temperature, and brain chemistry.

Linenger's experience provides the longest study to date of circadian rhythms in space. In the future, astronauts taking extended space trips could be prone to comparable changes that interfere with sleep and task performance, says psychologist Timothy H. Monk of the University of Pittsburgh School of Medicine, who directed the investigation.

While orbiting Earth every 2 hours aboard Mir, Linenger recorded data on himself for three blocks of time: days 37 to 50, days 79 to 91, and days 110 to 122. During these periods, Linenger took his oral temperature and rated his sense of alertness at the same five times during each 24-hour cycle. He also went to bed and arose at regular times in the cycle and recorded the amount and quality of his sleep.

As on Earth, Linenger's temperature declined as he fell asleep and rose toward the end of his slumber--at least for the first 91 days in space. During that time, the astronaut also reported sleeping well and staying alert while awake.

The situation later changed, however. In the third time block, Linenger displayed no daily cycle of temperature fluctuations. His body temperature remained nearly constant whether he slept or was awake. Moreover, he often didn't feel sleepy at his regular bedtime and felt as if he was losing track of each 24-hour day. His sleep time declined, and he awoke much more often than he had earlier in the flight.

Monk and his coauthors, including Linenger, present their results in the November/December PSYCHOSOMATIC MEDICINE.

Sleep loss and circadian disturbances also occurred among five astronauts who traveled for either 10 days or 16 days on the Space Shuttle in 1998, according to a report in the November AMERICAN JOURNAL OF PHYSIOLOGY. Astronauts' body temperature and secretion of the stress hormone cortisol stopped showing 24-hour cycles during the space flights, reports a team led by Charles A. Czeisler of Harvard Medical School in Boston. The shortened sleep periods included a marked decline in rapid-eye-movement sleep, which is associated with dreaming.

The crew members scored lower on attention and thinking tests in space than they had on Earth. It's unclear whether either sleep loss or circadian changes caused these declines--and Linenger's on Mir--Czeisler says.

"We may need to find ways to trick the circadian pacemaker into maintaining a 24-hour cycle when removed from Earth's time cues on long space missions," Monk says.