The sleep of four healthy young adult men was studied during a period of 20 days under bed rest conditions. The electroencephalograms (EEGs) were recorded during three 24-hour sessions (started from 21:00 to the subsequent day 21:00) on the 1st, 10th and 20th days of the bed rest period. Significant differences were found among the three measurement sessions in time length in stage II sleep, in rapid eye movement (REM) sleep and in total sleep. No differences were found in time length in stage I sleep, stage III + IV sleep and sleep latency. These results suggest that the sleep in healthy young adult men changed during the 20 days of bed rest.