Short Daytime Naps Briefly Attenuate Objectively Measured Sleepiness Under Chronic Sleep Restriction

Abstract

Study objectives: Napping is a useful countermeasure to the negative effects of acute sleep loss on alertness. The efficacy of naps to recover from chronic sleep loss is less well understood.

Methods: Following 2 baseline nights (10 hours' time-in-bed), participants were restricted to 7 nights of 5-hour sleep opportunity. Ten adults participated in the No-Nap condition, and a further 9 were assigned to a Nap condition with a daily 45-minute nap opportunity at 1300 h. Sleepiness was assessed using the multiple sleep latency test and a visual analogue scale at 2-hour intervals. Both objective and subjective indexes of sleepiness were normalized within subject as a difference from those at baseline prior to sleep restriction. Mixed-effects models examined how the daytime nap opportunity altered sleepiness across the day and across the protocol.

Results: Short daytime naps attenuated sleepiness due to chronic sleep restriction for up to 6-8 hours after the nap. Benefits of the nap did not extend late into evening. Subjective sleepiness demonstrated a similar short-lived benefit that emerged later in the day when objective sleepiness already returned to pre-nap levels. Neither measure showed a benefit of the nap the following morning after the subsequent restriction night.

Conclusions: These data indicate a short daytime nap may attenuate sleepiness in chronic sleep restriction, yet subjective and objective benefits emerge at different time scales. Because neither measure showed a benefit the next day, the current study underscores the need for careful consideration before naps are used as routine countermeasures to chronic sleep loss.

Keywords: MSLT; countermeasure; napping; sleep loss; sleep restriction; sleepiness.

Figure 1

Study protocol. Study protocol schematic illustrating the in-laboratory procedures. Each row indicates a day of the experimental protocol (BSLN: baseline day; R1-6: restriction days). Black shading denotes sleep episodes. Vertical gray bars indicate timing of multiple sleep latency test (MSLT); hashed bars indicate afternoon nap opportunity for participants in the Nap group. Vertical arrows illustrate half-hourly visual analogue scales (VAS) measuring subjective sleepiness.

Figure 2

MSLT across restriction days. MSLT-indexed sleep latencies across 6 days of the protocol for No Nap (black circles) and Nap (white circles), respectively. MSLT values expressed as differences from the baseline rested day (BSLN). Vertical lines indicate timing of afternoon nap. Error bars indicate standard error of mean (SEM). Pairwise t-tests at each MSLT test differences between group: *p < .05; **p < .01; ***p < .001.

Figure 3

MSLT across hours of the day. MSLT-indexed sleep latencies averaged across protocol days for No Nap (black circles) and Nap (white circles) groups, respectively. (A) Sleep latencies plotted for each time of day as differences from baseline, as in Figure 2. Vertical lines indicate timing of afternoon nap. (B) Sleep latencies collapsed into three segments, reported as marginal means from mixed-effects analysis reported in the manuscript. Error bars indicate standard error of mean (SEM). *p < .05.

Figure 4

Subjective sleepiness across hours of the day. Visual analogue scale (VAS)-measured subjective sleepiness averaged across protocol days for No Nap (black circles) and Nap (white circles) groups, respectively. (A) Sleepiness plotted for each time of day as differences from baseline, as in Figures 2 and 3A. Vertical lines indicate timing of afternoon nap. (B) Sleepiness collapsed into three segments, as in Figure 3B, reported as marginal means from mixed-effects analyses reported in the manuscript. Error bars indicate standard error of mean (SEM). *p < .05.