A number of studies have found that many subjects have reported being awake when awakened during various periods of electroencephalographically (EEG)-defined sleep. These observations have led to an examination of the perception of sleep after periods when EEG-defined sleep was experimentally varied between 1 and 140 min. Twelve normal young adult subjects slept in the laboratory for 5 consecutive nights. Each subject was briefly awakened five times on each night, and subjective state, auditory arousal threshold, and sleep/wake time estimation data were collected. The threshold of sleep onset (i.e., the point at which a report of sleep was given 50% of the time) occurred 2-4 min after the first sleep spindle. In contrast, auditory thresholds rose rapidly within 1 min of the first sleep spindle. The threshold data corroborate the appearance of a sleep spindle as an objective measure of sleep onset. However, subjective sleep onset appears to be a relatively lengthy period during which perception of state is blurred and uncertain.