Spontaneous termination of ad libitum sleep episodes with special reference to REM sleep

The spontaneous terminations of 72 ad libitum sleep episodes, recorded from 9 young adults, were analysed with respect to their proximity to REM sleep periods. Sleep episodes occurred throughout the 24 h day as a result of the experimental design, in which subjects were confined to bed for 60 continuous hours. For the group, sleep terminations occurred from REM sleep significantly more often than would be expected by chance (40% of the time). However, substantial individual differences in 'style' of sleep termination were observed, with 4 subjects contributing 82% of all terminations from REM sleep. Such individual differences were noted also in the tendency to exhibit brief interruptions of ongoing REM episodes. Both terminations of sleep from REM and brief interruptions of REM showed a general tendency to occur prior to about halfway through a REM sleep episode. The tendency to awaken differentially from REM sleep is interpreted on the basis of REM sleep providing the optimal physiological conditions for the transition from sleep to waking.