A few studies have reported that nighttime sleep and daytime napping are significantly associated with sleep disorders, memory, concentration, cognitive ability, and academic performance in children. Moreover, sleep is identified as an important factor that has a profound effect on children’s athletic performance. If a short daytime nap is shown to benefit children involved in sports activities, this evidence could be used to improve children’s athletic performance in the afternoon. In this study, we investigated whether a short daytime nap could reduce the decline in athletic performance observed in the afternoon in children involved in basketball games. We investigated 10 elementary school-age male basketball players. The study was performed under two conditions, with and without a 20-min nap during the lunch break, and evaluation was performed under both conditions for 3 days each. The reaction and 20-m sprint times were measured after morning practice, before afternoon practice, and at the end of practice on all days. At the beginning of afternoon practice, the 20-m sprint time was significantly shorter (p<0.05) in the no-nap group (4.18±0.27 s) than in the nap group (4.24±0.24 s). In conclusion, our results suggest that a short daytime nap in children may reduce the decline in exercise performance observed in the afternoon.