Highly relevant stimuli may passively elicit processes associated with consciousness during the sleep onset period.

Sleep onset marks the transition from waking to sleep, during which conscious awareness of the external environmental is gradually lost. The present study examines the extent of processing of acoustic change during sleep onset. An auditory optimal paradigm was used to record event-related potentials to six deviant stimuli during wakefulness, stage N1, and stage N2 sleep. During waking and early-stage N1, two of the deviants, environmental sounds and white noise, elicited a P3a reflecting processes that may lead to conscious awareness of acoustic change. Surprisingly, the P3a was also observed following both deviants during late-stage N1, a period thought to represent decreased awareness of the environment. Only the environmental sounds continued to elicit a P3a during stage N2 sleep, associated with the loss of consciousness of the external environment. Certain auditory stimuli may thus continue to activate processes that may lead to conscious awareness during the sleep onset period.