While short daytime naps have been found to provide alertness and performance benefits without inducing sleep inertia, the effects of a similar napping strategy during nights shifts are largely unknown. This study examined the effects of a 20-min nap (scheduled at 2:00 a.m.) during night shifts among 16 participants (mean [SD] age 22.0 [1.08] years) who worked in a simulated night shift from midnight (12:00 a.m.) to morning (ending at 6:00 or 8:00 a.m.). The participants underwent both a ‘nap condition’ and a ‘no-nap condition’ by engaging in repeated 10-min tasks (four-choice reaction time tasks and vigilance tasks) and 10-min rest. The results showed that compared to the no-nap condition, sleepiness was significantly lower in the nap condition between 3:20 and 5:20 a.m. (p < 0.05). The nap condition also yielded significantly better performance in the vigilance tasks between 2:40 and 5:40 a.m., except at 4:40 am (p < 0.05). However, no significant difference was found in sleepiness and performance in the vigilance task at 5:40 a.m. or immediately after the nap between the two conditions. The results showed that a 20-min nap at 2:00 a.m. did not induce sleep inertia upon waking. Furthermore, it mitigated sleepiness and sustained work performance for 3 h after the nap. However, the effect of napping was no longer observed near the end of the night shift. Considering risks of falling asleep while driving home from work that may cause a traffic accident, further research should examine ways to mitigate sleepiness after a night shift.