Relations among hypnagogic and hypnopompic experiences associated with sleep paralysis.

The Waterloo Sleep Experiences Scale was developed to assess the prevalence of sleep paralysis and a variety of associated hypnagogic and hypnopompic hallucinoid experiences: sensed presence, felt pressure, floating sensations, auditory and visual hallucinations, and fear. Consistent with results of recent surveys, almost 30% of 870 university students reported at least one experience of sleep paralysis. Approximately three-quarters of those also reported at least one hallucinoid experience, and slightly more than 10% experienced three or more. Fear was positively associated with hallucinoid experiences, most clearly with sensed presence. Regression analyses lend support to the hypothesis that sensed presence and fear are primitive associates of sleep paralysis and contribute to the elaboration of further hallucinoid experiences, especially those involving visual experiences.