During natural nocturnal sleep, Na and Pa middle latency auditory evoked potentials (MLAEPs) underwent significant variations which were related to sleep stages. The deepening of sleep from stage II to stage IV was paralleled by a latency shift and an amplitude decrease of Na and Pa, while MLAEPs were similar in wakefulness and REM sleep. Moreover, Na and Pa components tended to shift over the hemisphere contralateral to the stimulated ear during sleep. These findings demonstrate that the responsiveness of the auditory cortex to acoustic stimuli is modulated during sleep. Vigilance should be monitored during MLAEP recordings in patients.