0300 The Circadian Timing of Sleep Affects the Rate of Accumulation of Neurobehavioral Impairment Across Days of Sleep Restriction

Introduction Chronic restriction of nighttime sleep to less than ~8h/day leads to build-up of neurobehavioral impairment across days. Although it is known that sleep loss effects depend on the circadian timing of sleep, it is not known how the timing of restricted sleep influences the accumulation of neurobehavioral impairment over days. Here we studied the accumulation of impairment across days of restricted sleep placed in the morning or afternoon. Methods N=71 healthy young adults (39% female; ages 21-45y, mean±SD: 27.9±6.6y) completed a 14-day laboratory study. After two baseline days with nighttime sleep (8h TIB: 23:30-07:30), subjects were randomized to 10 consecutive days of A) morning sleep at 4h, 6h, or 8h TIB ending at 11:30 each day (n=18, 8, 8, respectively), or B) afternoon sleep at 4h, 6h, or 8h TIB ending at 19:30 each day (n=13, 17, 7, respectively). Subjects were tested on the 10min psychomotor vigilance test (PVT) every ~2 hours during scheduled wakefulness. Daily averages for PVT lapses (RTs>500ms) observed between 2h and 14h after awakening were analyzed with non-linear mixed-effects regression to investigate differences in the neurobehavioral impairment build-up rate between sleep restriction conditions. Results Afternoon sleep conditions showed a significant sleep dose-response effect (p<0.001), with the fastest accrual of PVT performance deficits across days in the 4h condition, and slow-to-negligible accumulation (p=0.36) of PVT performance deficits in the 8h condition. However, morning sleep resulted in no significant sleep dose-response effect (p=0.96). All 3 morning sleep doses displayed negligible (p≥0.12) accumulation of impairment across days. Conclusion In this sample of young adults, sleep dosages ending in the morning (at 11:30) appear to provide considerable protection against cumulative performance deficits from sleep restricted to 4h-6h/day over 10 days, suggesting that the afternoon circadian promotion of wakefulness can sustain behavioral alertness even over multiple days of repeated sleep restriction. Support NIH grants R01-NR04281 and M01-RR00040