The present study examined the effects of sleep inertia on arousal level and frontal lobe function after a normal night's sleep. Nine undergraduate and graduate students slept for two consecutive nights (adaptation and experimental). The bedtime and waking times were fixed as the times that were usual for the participants in their daily life. A switching task that reflects frontal lobe functioning and a simple auditory reaction time task that reflects general arousal level were conducted for 15 min before bedtime and for 60 min immediately after waking. In comparison with 15 min before bedtime, the arousal level was lowered for 15 min and subjective sleepiness deteriorated for 1 h after awakening, whereas performance on the switching task was not reduced after awakening. These results suggest that sleep inertia occurring after a normal night's sleep affects arousal level but does not have an appreciable effect on frontal lobe functions.