The correlation between the effects of zopiclone (ZPC), a nonbenzodiazepine hypnotic drug, on sleep polysomnograms and on daytime EEGs was examined in 12 healthy adult male volunteers. Sleep polysomnograms were recorded after a single oral administration of ZPC 10 mg or placebo according to the double-blind crossover method. Daytime EEGs were recorded after the administration of ZPC, 7.5 mg, or placebo in the same manner, and recorded for 3 min with closed eyes at rest. Then, square roots of the absolute power (amplitude) of the δ-, θ-, α-, and β-activities were calculated from the power spectrum obtained by the fast Fourier transform method. As a result, ZPC decreased the percentage of stage 1 sleep in total sleep time, while it increased the percentage of stage 2, total sleep time, and time of slow wave sleep in the first and second sleep cycles (SWS 1 and 2). Changes in SWS 1 and 2 correlated positively with the amplitude changes in daytime resting δ-activity. This indicates that the increase of SWS due to ZPC could be related to the changes of δ-activity in the daytime resting