This study examined the relationships between behaviorally signaled awakenings (BSAs) and sleep stage or NREM/REM cycle during nocturnal sleep in humans. Polysomnogram was recorded in the laboratory from five adult males and five adult females for six successive nights. They were instructed to press a button four times (behaviorally signaled awakenings; BSAs) whenever they perceived their own awakenings from sleep. The results indicated that BSAs occurred more preferentially from REM sleep than from NREM sleep, that 55.1 % of BSAs from REM sleep terminated REM sleep, and that BSAs from NREM sleep tended to occur more frequently as nearing the onset of REM sleep. It is suggested that REM sleep is a state in which spontaneous awakenings preferentially occur during nocturnal sleep and that the period of transition between REM and NREM sleep is the time when behaviorally spontaneous awakenings from sleep are most likely.