We assessed the effects of the length of sleep prior to awakening on the sense of having been asleep in 11 normal, healthy, young adults. They were awakened a total of 194 times out of stage 2 sleep, 153 times out of REM sleep, and 13 times out of slow-wave sleep over 54 nights in the laboratory. At each awakening, subjects were asked if they had been awake or asleep prior to the arousal. Continuous sleep time prior to the awakening was calculated in minutes to the first visually scorable alpha intrusion exceeding 10 s on either the central or occipital electroencephalogram (EEG) tracings. The sense of having been asleep prior to a NREM sleep awakening is dependent on the length of continuous, prior sleep time. However, the sense of having been asleep when aroused out of REM sleep shows no relationship to the length of continuous, prior sleep time.