The cumulative cost of additional wakefulness

Objectives: To inform the debate over whether human sleep can be chronicall y reduced without consequences, we conducted a dose-response chronic sleep restriction experiment in which waking neurobehavioral and sleep physiologi cal functions were monitored and compared to those for total sleep deprivat ion. Design: The chronic sleep restriction experiment involved randomizatio n to one of three sleep doses (4 h, 6 h, or 8 h time in bed per night), whi ch were maintained for 14 consecutive days. The total sleep deprivation exp eriment involved 3 nights without sleep (0 h time in bed). Each study also involved 3 baseline (pre-deprivation) days and 3 recovery days. Setting: Bo th experiments were conducted under standardized laboratory conditions with continuous behavioral, physiological and medical monitoring. Participants: A total of n = 48 healthy adults (ages 21-38) participated in the experime nts. Interventions: Nocturnal sleep periods were restricted to 8 h, 6 h or 4 h per day for 14 days, or to 0 h for 3 days. All other sleep was prohibit ed. Results: Chronic restriction of sleep periods to 4 h or 6 h per night o ver 14 consecutive days resulted in significant cumulative, dose-dependent deficits in cognitive performance on all tasks. Subjective sleepiness ratin gs showed an acute response to sleep restriction but only small further inc reases on subsequent days, and did not significantly differentiate the 6 h and 4 h conditions. Polysomnographic variables and delta power in the non-R EM sleep EEG-a putative marker of sleep homeostasis-displayed an acute resp onse to sleep restriction with negligible further changes across the 14 res tricted nights. Comparison of chronic sleep restriction to total sleep depr ivation showed that the latter resulted in disproportionately large waking neurobehavioral and sleep delta power responses relative to how much sleep was lost. A statistical model revealed that, regardless of the mode of slee p deprivation, lapses in behavioral alertness were near-linearly related to the cumulative duration of wakefulness in excess of 15.84 h (s.e. 0.73 h). Conclusions: Since chronic restriction of sleep to 6 h or less per night p roduced cognitive performance deficits equivalent to up to 2 nights of tota l sleep deprivation, it appears that even relatively moderate sleep restric tion can seriously impair waking neurobehavioral functions in healthy adult s. Sleepiness ratings suggest that subjects were largely unaware of these i ncreasing cognitive deficits, which may explain why the impact of chronic s leep restriction on waking cognitive functions is often assumed to be benig n. Physiological sleep responses to chronic restriction did not mirror waki ng neurobehavioral responses, but cumulative wakefulness in excess of a 15. 84 h predicted performance lapses across all four experimental conditions, This suggests that sleep debt is perhaps best understood as resulting in ad ditional wakefulness that has a neurobiological cost which accumulates over time.