Duration of Sleep Inertia after Napping during Simulated Night Work and in Extended Operations

Abstract

Due to the mixed findings of previous studies, it is still difficult to provide guidance on how to best manage sleep inertia after waking from naps in operational settings. One of the few factors that can be manipulated is the duration of the nap opportunity. The aim of the present study was to investigate the magnitude and time course of sleep inertia after waking from short (20-, 40- or 60-min) naps during simulated night work and extended operations. In addition, the effect of sleep stage on awakening and duration of slow wave sleep (SWS) on sleep inertia was assessed. Two within-subject protocols were conducted in a controlled laboratory setting. Twenty-four healthy young men (Protocol 1: n = 12, mean age = 25.1 yrs; Protocol 2: n = 12, mean age = 23.2 yrs) were provided with nap opportunities of 20-, 40-, and 60-min (and a control condition of no nap) ending at 02:00 h after ∼20 h of wakefulness (Protocol 1 [P1]: simulated night work) or ending at 12:00 h after ∼30 h of wakefulness (Protocol 2 [P2]: simulated extended operations). A 6-min test battery, including the Karolinska Sleepiness Scale (KSS) and the 4-min 2-Back Working Memory Task (WMT), was repeated every 15 min the first hour after waking. Nap sleep was recorded polysomnographically, and in all nap opportunities sleep onset latency was short and sleep efficiency high. Mixed-model analyses of variance (ANOVA) for repeated measures were calculated and included the factors time (time post-nap), nap opportunity (duration of nap provided), order (order in which the four protocols were completed), and the interaction of these terms. Results showed no test x nap opportunity effect (i.e., no effect of sleep inertia) on KSS. However, WMT performance was impaired (slower reaction time, fewer correct responses, and increased omissions) on the first test post-nap, primarily after a 40- or 60-min nap. In P2 only, performance improvement was evident 45 min post-awakening for naps of 40 min or more. In ANOVAs where sleep stage on awakening was included, the test x nap opportunity interaction was significant, but differences were between wake and non-REM Stage 1/Stage 2 or wake and SWS. A further series of ANOVAs showed no effect of the duration of SWS on sleep inertia. The results of this study demonstrate that no more than 15 min is required for performance decrements due to sleep inertia to dissipate after nap opportunities of 60 min or less, but subjective sleepiness is not a reliable indicator of this effect. Under conditions where sleep is short, these findings also suggest that SWS, per se, does not contribute to more severe sleep inertia. When wakefulness is extended and napping occurs at midday (i.e., P2), nap opportunities of 40- and 60-min have the advantage over shorter duration sleep periods, as they result in performance benefits ∼45 min after waking.

Waking up is the hardest thing I do all day: Sleep inertia and sleep drunkenness

“…This is thought to reflect increased sleep pressure following sleep deprivation, suggesting that SWS amount during sleep should correlate with cognitive impairment upon awakening [44]. Evidence in support of this hypothesis has been mixed, with some [44, 45], but not all [46–48], studies demonstrating a relationship between SWS amount or percentage and waking cognitive performance (Table 2). Relative little attention has been paid to sleep inertia in children, but nocturnal SWS awakenings impair their behavioral responsiveness considerably [43].…”

“…After nocturnal time in bed of 5 hours, daytime nap sleep times up to 20 minutes do not convey sleep inertia effects, while sleep inertia is clearly apparent after 30 minutes of measured sleep [49–51]. However, under more pronounced sleep deprivation, naps shorter than 30 minutes time in bed may sometimes result in sleep inertia [16, 46, 52]. …”

Time to wake up: reactive countermeasures to sleep inertia

“…Much of this research has focussed on establishing the best length and timing of sleep to minimise sleep inertia. Studies suggest that avoiding sleep periods ending during the circadian low5, 15 ) , and keeping naps to less than 30 min4, 16, 17, 18 ) to avoid waking from deep sleep15 ) can minimise sleep inertia magnitude. Prior sleep loss can also exacerbate sleep inertia15, 19 ) , which is particularly important in shiftwork where workers often experience extended wakefulness and/or sleep loss20, 21 ) .…”

“…This study demonstrates that, when investigating sleep inertia countermeasures, it is important to use experimental circumstances that are most likely to produce sleep inertia. For example, following: scheduled awakenings at night5, 15 ) ; extended wakefulness or prior sleep loss15, 19 ) ; or longer naps4, 16, 18 ) .…”

“…A clue to its role comes from the discovery that SWS is associated with lower cortisol75 ) . Deep sleep has also been associated with greater sleep inertia15, 76 ) , although this relationship has not been consistently demonstrated18, 77 ) . The relationship between deep sleep and sleep inertia may be mediated by cortisol, although this has not been directly investigated.…”