Previous studies have demonstrated that motor activity during sleep is lateralized to the nondominant hand. There are two basic theories concerning this phenomenon: 1) The nondominant hemisphere is nonspecifically more alert or responsive than the dominant one, and 2) the lateralization to the nondominant side is task specific, reflecting the spatially oriented mode of information processing that is responsible for movements during sleep. We examined the motor responses to auditory stimuli during waking and sleep of 10 right-handed healthy subjects, who were instructed to switch off a tone stimulus by pressing a transducer that was attached to each hand. Sleep stage scoring was performed according to Rechtschaffen and Kales's criteria. During wakefulness and in all stages of sleep, with and without alpha activity occurring after stimulus onset, the dominant hand was used more, but during nonrapid eye movement S1 sleep the difference was not statistically significant. When alpha activity was present in the electroencephalogram after stimulus onset, the responses were significantly more lateralized to the right hand than when there was no alpha activity. During an actimetric home recording of both wrists of the subjects, there was an excess of left-sided movements during sleep as compared to waking. The results do not support the idea that the right hemisphere is generally more responsive than the left during sleep. They are, however, in accordance with the hypothesis that spatial information processing is a crucial factor in the nondominant lateralization of spontaneous sleep movements.(ABSTRACT TRUNCATED AT 250 WORDS)