A short daytime nap of less than 30 min can prevent later sleepiness. However, severe sleep inertia often occurs after awakening from slow–wave sleep (SWS). The present study examined the appearance pattern of SWS during daytime hours. Polysomnograms of university students (n = 101) showed that SWS did not appear during a nap of less than 15 min, but did appear at least 9 min after the first sleep spindle. These results suggest that a short nap (less than 15 min) does not produce sleep inertia.