Systematic Individual Differences In Vulnerability To Sleep Inertia

Abstract Introduction Sleep inertia (SI), the transient grogginess and disorientation that occurs upon awakening, may be particularly problematic in on-call operations that require safety-sensitive and time-critical action, such as healthcare and emergency response. The magnitude of SI is determined by multiple factors, including sleep history, and anecdotal evidence suggests individuals may differ considerably in susceptibility to impairment from SI. Methods As part of a larger study investigating individual differences in neurobehavioral impairment, N=21 healthy adults (aged 21–38y; 9 females) completed three laboratory-based neurobehavioral testing sessions, each preceded by baseline sleep. Baseline sleep and nightly at-home sleep opportunities during the week prior were either 12h (extended) or 6h (restricted); two sessions involved extension and one involved restriction in randomized, counterbalanced order. Baseline sleep opportunities ended at 10:00, whereupon subjects completed a 60min neurobehavioral test battery, which began with the Karolinska Sleepiness Scale (KSS). The test battery was repeated every 2h throughout the testing sessions. Results A nonlinear mixed-effects regression, controlling for prior sleep restriction/extension and session number, was used to estimate the subjective magnitude of SI immediately upon awakening from baseline sleep, as measured by KSS scores, and the exponential dissipation rate of the effect relative to KSS scores later in the day (12:00-20:00). Following prior sleep extension, SI was associated with a 1.82±0.59 KSS score increase (p=0.006), which subsequently dissipated from a level of 4.80±0.65 to 2.98±0.29 (p=<0.001) later in the day. Following prior sleep restriction, SI was associated with a 1.58±0.58 KSS score increase (p=0.014), which subsequently dissipated from a level of 5.39±0.63 to 3.80±0.30 later in the day. SI took ~45min to dissipate to a negligible level based on a time constant estimate of 23.6±15.8min. Importantly, there were substantial, systematic individual differences in the magnitude of SI, with a between-subjects standard deviation of 1.15±0.42 points on the KSS (ICC=0.51, F=19.5, p<0.001). Conclusion We observed sizeable, systematic individual differences in subjective sleepiness due to sleep inertia. To what degree these individual differences predict objective performance deficits remains to be investigated. Support (if any) NASA grant NAG9-1161 and CDMRP grant W81XWH-20-1-0442