Objective : To investigate sleepiness, fatigue, and performance following a 120-minute nap during simulated 16-hour night shifts based on subjective and objective assessments.

Methods : Fourteen females participated in this crossover comparative study. Three experimental nap conditions were used: naps from 22:00 to 00:00 (22-NAP), 00:00 to 02:00 (00-NAP), and 02:00 to 04:00 (02-NAP), respectively. Measurement items were sleep parameters, sublingual temperature, a Visual Analog Scale for sleepiness and fatigue, a single-digit mental arithmetic task (for 10 minutes), and heart rate variability. Participants wore an ActiGraph to estimate their sleep state.

Results : There was no difference in the sleep parameters at the time of naps among the three conditions. Immediately following a 120-minute nap, sleepiness and fatigue increased, and the number of calculations performed in the single-digit mental arithmetic task decreased in any of the conditions. In particular, immediately after the 02-NAP, fatigue and high-frequency power (HF) were higher than after the 22-NAP. In the early morning (from 05:00 to 09:00), in the 22-NAP, sleepiness and fatigue increased, and performance and sublingual temperature decreased more than in the 00-NAP and 02-NAP. Furthermore, the ratio of errors was significantly lower in the 00-NAP than in the 22-NAP in the early morning.

Conclusions : A 120-minute nap taken from 22:00 to 02:00 may cause temporary sleepiness after waking, increase fatigue and reduce performance. Greater attention should be given to naps taken at a later time (ie, 02-NAP). In addition, taking a nap starting at 00:00 might decrease the risks of errors in the morning.