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Sleep problems have been observed during many of the space flights. The existence of poor quality of sleep, fatigue, insomnia or different alterations in sleep structure, organization and sleep cyclicity have been established. Nevertheless results obtained from investigations of human sleep on board manned space vehicles show that it is possible to keep sleep patterns related to the restorative and adaptive processes. For the first time in the frame of the "Intercosmos" program a multi-channel system for recording and analysis of sleep in space was constructed by scientists of the Bulgarian Academy of Sciences and was installed on board the manned Mir orbiting station. In 1988 during the joint Bulgarian-Russian space flight continues recording of electro-physiological parameters necessary to estimate the sleep stages and sleep organization was made. These investigations were continued in next space flights of different prolongation. The results were compared with the findings obtained under the conditions during the pre- and post-flight periods., (c2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.)

In 40 aquanauts, a prolonged stay under increased pressure (11 to 46 kgs/cm2) of the oxygen-helium-nitrogen mixture did not affect the average duration of sleep. Slow-wave sleep, mostly its 3 rd and 4 th stages, and paradoxical sleep were significantly decreased whereas the light sleep/profound sleep ratio increased. The cyclic structure of sleep became altered. The longer the exposure to high pressure led to an augmentation of the slow-wave sleep and REM-phase, but the normal cycles terminating with a REM-phase could not be formed during the experiment.