In order to clarify the functional asymmetry of the brain function during sleep, period-amplitude analysis of delta electroencephalogram activity was performed on polysomnograms in 12 right-handed healthy males. Electroencephalograms were recorded from disc electrodes placed at C3, C4, O1 and O2 (10-20 electrode system), using A1 +A2 for reference. Although there were no significant differences in delta counts between O1 and O2, delta counts of C3 were significantly larger than those of C4. These results suggest that there exists distinct laterality in the number of delta waves in the central region, reflecting the functional asymmetry of the brain during sleep.