In a group of 6 male subjects sleep was displaced to seven different times of day (one displacement condition per week). The subjects were isolated from external time cues (daylight, clocks, noise) and sleep was allowed to terminate spontaneously. The results showed a pronounced time-of-day variation of total sleep time, stage 2, and rapid eye movement (REM) sleep. Maxima occurred after bedtimes at 1900 hr and 2300 hr, while the minima occurred after bedtimes at 0700 hr and 1100 hr. The latter also was the time of maximum propensity to wake up. Slow wave sleep showed a rapid decrease from high initial levels, irrespective of time of day. Ratings of sleepiness showed a highly significant circadian variation peaking between 0500 hr and 0700 hr. The lowest level of sleepiness coincided with the maximum tendency to wake up, and it was suggested that sleep termination may be closely related to the sleepiness/ alertness rhythm.