A short nap of less than 30 min is an effective countermeasure to prevent afternoon sleepiness and resulting sleep-related accidents. This study was designed to confirm the effectiveness of napping on a car seat equivalent, and to compare two different backrest angles that are preferred by drivers. Eleven healthy university students (mean age 21.3 years, SD = 1.7) completed three experimental conditions: no-nap with the backrest positioned at 105°, and two 15-min naps, with the backrest inclined at 130° or 150°. Both naps with a 130° and 150° backrest incline reduced subjective sleepiness, improved task performance and suppressed slow eye movements (indicators of drowsiness) during the vigilance task. Napping with a backrest angle of 150° reduced subjective fatigue and improved reaction time further than that seen for naps at 130°. These results suggest that a short nap of 15 min in a car seat can counteract afternoon sleepiness and improve task performance. These effects are enhanced when the backrest angle is closer to the horizontal position.