Information processing of auditory stimuli in sleeping healthy volunteers is studied by means of an oddball paradigm. Latencies of the early (N1P2) complex are affected by NREM sleep depth and show a progressive slowing. The amplitude of the early N1 component depends on stimulus type and probability and is increased following deviant stimuli. In strict comparison with waking, late responses (N2-P3) complex only follows deviant stimuli. Both latencies and amplitude increase more in NREM than in REM sleep. Thus the early response is mainly affected by vigilance, the late response mainly depends on stimulus type. Results are discussed in terms of comparison processing, context updating and orienting response patterning hypotheses put forward in cognitive psychophysiology.