The amount of wakefulness and slow wave sleep (SWS) during the 12-h light period slightly but significantly decreased and increased, respectively, in freely behaving rats after continued 1-week intake of Panax ginseng extract through drinking water (15 mg/day). Paradoxical sleep was little affected. No sleep parameters were modulated by the treatment during the dark period. The diurnal SWS enhancement disappeared and recovered to the baseline level after 2 weeks of continued treatment. It is speculated that the well known health-improving effect of the ginseng may be, at least in part, related to an enhancement of sleep.