Discrimination of sleep onset stages: behavioral responses and verbal reports

In a descriptive experiment on discrimination five human subjects were studied during the transitional period of sleep onset. Subjects were aroused by an abrupt auditory stimulus, attempted to discriminate the pre-arousal stage by a behavioral response, and answered a series of standardized questions. These questions focused on specific characteristics of private experience associated with sleep onset. Of 180 awakenings, subjects correctly identified 109 sleep-onset stages. Subjects' answers were analyzed to determine what criteria were used to make the discrimination among sleep-onset stages and to examine their self-awareness of changes in private experience. It was established that there are stage-related changes in mental processes and content and that these may aid subjects in making such discriminations. Implications of the methodological approach used in this study are discussed.