The effects of a scheduled nap during the night shift on sleepiness and cognition in hospital nurses

Sleepiness and cognitive performance were compared with and without a 30-minute nap scheduled at 4:00 a.m. during the 8-hour night shift (23:00-07:00). All participants were tested on two nights with and two nights without a nap. Maximum two consecutive nights were allowed, all participants completed 4 study nights within 2 weeks period. To minimize the effect of order, participants were randomly assigned to five order groups: (1) nap/no-nap/nap/no-nap, (2) no-nap/nap/no-nap/nap, (3) nap/no-nap/no-nap/nap, (4) no-nap/nap/nap/no-nap, and (5) no-nap/no-nap/nap/nap. During the night shifts, participants reported hourly on sleepiness and performed two cognitive tests, the Letter Cancellation Task (LCT) and the Digit Symbol Substitution Task (DSST), at 3:00 and 7:00 a.m. On nights with a scheduled nap, participants were instructed to retire to a dark and quiet room for 40 minutes at 4:00 a.m. (corresponding to the nadir of circadian alertness, Borbély, 1982), to allow for an approximately 30-minute nap and an additional 10 minutes both to settle down before and to recover after the nap. On no-nap nights they continued their work as usual. To monitor sleep duration and time awake since last sleep, participants wore an actigraph 24 hours before and during the night shift. At the end of each night shift, workload, unusual events (if any occurred), and number of caffeinated beverages consumed were recorded and perceptual nap efficiency was assessed. Participants completed a sociodemographic questionnaire, the Munich ChronoType Questionnaire for Shiftwork (MCTQShift), Pittsburgh Sleep Quality Index (PSQI), and Pre-Sleep Arousal Scale (PSAS) between shifts. Study research assistants were on site to ensure adherence to the nap protocol and completion of the performance tests and study questionnaires.