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Abstract Introduction Night shift workers experience increased sleepiness during the night due to increasing homeostatic sleep drive and reduced circadian wake drive, and because their daytime sleep is typically restricted. Nighttime napping is a countermeasure to sleepiness, but the effectiveness of different amounts of nighttime nap sleep in night shift schedules with restricted daytime sleep has not been systematically investigated. Methods N=79 healthy adults (ages 28.5±6.8y; 53 males) participated in a laboratory study with 8 days of simulated night shift. Subjects were randomized to one of 18 daily sleep regimens involving restricted daytime sleep (4.2h, 5.2h, 6.2h, or 8.2h TIB centered at 14:00) combined with a nighttime nap (0h, 0.4h, 0.8h, 1.2h, 1.6h, 2.0h, or 2.4h TIB centered at 02:00). Every day at 20:10, 22:10, 00:10, 04:10, 06:10, 08:10 (all post sleep inertia), subjective sleepiness and vigilant attention were assessed with the Karolinska Sleepiness Scale (KSS) and Psychomotor Vigilance Test (PVT), respectively. For each subject, circadian change was determined as the difference between the daily peak and trough of KSS scores and PVT lapses (RT500ms). Daily peak sleepiness and circadian change were analyzed using mixed-effects regression with fixed effects for nighttime nap TIB (categorical), night shift day, and their interaction; covariates for daytime sleep TIB (categorical) by itself and in interaction with day; and a random effect over subjects on the intercept. Results Nap duration influenced KSS daily peak (F6,543=2.25, P=0.037) and circadian change (F6,543=3.18, P=0.005), with 0h TIB (no nap) yielding the highest peak and circadian change (P 0.05), 0.8h TIB the lowest (P 0.05), and no significant differences comparing the other nap durations. Nap duration did not significantly affect PVT daily peak impairment (P=0.36) or circadian change (P=0.16). Conclusion Nighttime napping of any duration (0.4h–2.4h TIB) centered at 02:00 has the potential to mitigate peak subjective sleepiness during night shift schedules with restricted daytime sleep. This napping effect does not appear to extend to impairment in objectively measured vigilant attention. More research is needed to understand the extent to which performance and safety benefit from nighttime napping under conditions of sustained daytime sleep restriction during night shift schedules. Support (if any) NASA NCC 9-58-159 with NSBRI
Muck et al. (Sat,) studied this question.