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Principal Investigator:
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David F. Dinges, Ph.D.
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Organization:
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University of Pennsylvania
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Project Ended:
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2000
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This project is concerned with identifying ways to prevent neurobehavioral and physical deterioration due to inadequate sleep in astronauts during long-duration manned spaceflight. The performance capability of astronauts during extended-duration spaceflight depends heavily on achieving recovery through adequate sleep. Even with appropriate circadian alignment, sleep loss can erode fundamental elements of human performance capability including vigilance, cognitive speed and accuracy, working memory, reaction time, and physiological alertness. Adequate sleep is essential during manned spaceflight not only to ensure high levels of safe and effective human performance, but also as a basic regulatory biology critical to healthy human functioning.
There is now extensive objective evidence that astronaut sleep is frequently restricted in spaceflight to averages between 4 hr and 6.5 hr/day. Chronic sleep restriction during manned spaceflight can occur in response to endogenous disturbances of sleep (microgravity, motion sickness, stress, circadian rhythms), environmental disruptions of sleep (noise, temperature, light), and curtailment of sleep due to the work demands and other activities that accompany extended spaceflight operations. The mechanism through which this risk emerges is the development of cumulative homeostatic pressure for sleep across consecutive days of inadequate sleep. Research has shown that the physiological sleepiness and performance deficits engendered by sleep debt can progressively worsen (i.e., accumulate) over consecutive days of sleep restriction, and that sleep limited to levels commonly experienced by astronauts (i.e., 4 - 6hr per night) for as little as 1 week, can result in increased lapses of attention, degradation of response times, deficits in complex problem solving, reduced learning, mood disturbance, and disruption of essential neuroendocrine functions.
The prevention of cumulative performance deficits and neuroendocrine disruption from sleep restriction during extended duration spaceflight involves finding the most effective ways to obtain sleep in order to maintain the high-level cognitive and physical performance functions required for manned spaceflight. There is currently a critical deficiency in knowledge of the effects of how variations in sleep duration and timing relate to the most efficient return of performance per unit time invested in sleep during long-duration missions, and how the nature of sleep physiology changes as a function of sleep restriction and performance degradation. The primary aim of this project is to meet these critical deficiencies through utilization of a response surface experimental paradigm, testing in a dose-response manner, varying combinations of sleep duration and timing, for the purpose of establishing how to most effectively limit the cumulative adverse effects on human performance and physiology of chronic sleep restriction in space operations.
To develop a response surface models of the best sleep-wake schedules for astronauts, 90 healthy men and women underwent a 14-day ground-based laboratory protocol involving random assignment to one of 18 sleep-ration cells, each involving the same sleep ration for 10 consecutive days. The sleep-ration assignments involved four nocturnal anchor sleep durations (4.2, 5.2, 6.2, 8.2 hr) and six diurnal nap sleep durations (0.4, 0.8, 1.2, 1.6, 2.0, 2.4 hr) crossed to yield a total of four anchor-sleep-only conditions, and 14 anchor + nap sleep conditions, and spanning a dynamic range of cumulative sleep debts (i.e., from 0 to 40 hr in a 10-day period). Throughout the 14 days, subjects lived in conditions that simulated aspects of prolonged spaceflight (e.g., confined small groups, social and environmental isolation, controlled diet and activity) and underwent a wide range of quasi-continuous neurobehavioral performance tests and continuous physiological monitoring of brain activity, sleep physiology, core body temperature, and behavioral motility. The laboratory environment was designed to simulate the low light, tight quarters, and lack of social contact with the outside world that will characterize long-duration spaceflight.
Data acquisition in this project has been completed response surface model (RSM) development and hypothesis testing on the large set of neurobehavioral and physiological outcomes are currently underway. Thus far, the results of the experiment have revealed that subjects are able to achieve significant levels of physiological sleep when both anchor (nocturnal) and nap (diurnal) sleep opportunities are chronic (i.e., part of a daily schedule), regardless of the time in bed allowed for sleep. Even daytime nap opportunities as brief as 0.4 hr (24 minutes) consistently resulted in physiological sleep. This indicates that the diverse range of restricted anchor + nap sleep durations tested in this protocol will likely result in sleep if used by astronauts. Response surface models fit to neurobehavioral data reveal that the combination of a restricted duration anchor sleep and a diurnal nap can help prevent the development of cumulative deficits that can occur when only restricted anchor sleep is permitted (as is currently typical in spaceflight). Moreover, all response surface models being evaluated for optimizing performance, mood, sleep physiology, and hormonal profiles in the face of restricted total sleep time include the duration of nocturnal anchor sleep, the duration of diurnal nap sleep, age, gender, and baseline individual differences. Thus, the results of this study will permit us to estimate the relative contributions of astronaut demographics (age, gender) and individual abilities (baseline differences) to cumulative neurobehavioral deficits due to sleep restriction. This permits a more precise estimate of how a given sleep-wake schedule will likely affect different crews.
This experiment is the first ground-based study to utilize the slopes of cumulative neurobehavioral deficits and physiological changes across days of chronic sleep restriction, to determine the extent to which the duration of sleep per 24 hours (in the range commonly experienced by astronauts in flight) and the use of combined anchor + nap sleep opportunities each day, can prevent or attenuate the development of cumulative fatigue and performance deficits. The response surface experimental paradigm affords a high return of information regarding the optimal way to utilize sleep in operations that inherently limit time for sleep in the spaceflight environment. The results of the proposed research will contribute to the optimization of performance, productivity, safety and health during extended missions, by providing astronauts with the most efficient sleep-wake schedules. The results of this project also have important implications for optimizing work-rest scheduling in Earth-based safety-sensitive industries that must operate around the clock (e.g., transportation, military, public safety).
Finally, the research project will also help address critical questions pertaining to human performance failure in space due to sleep and circadian problems. Thus, the results of this project will help determine both the acute and long-term neurobehavioral and physiological effects of exposure to restricted sleep durations in the range commonly experienced by astronauts in spaceflight. It will establish whether sleep-wake schedule countermeasures involving varying combinations of restricted anchor sleep and nap sleep can effectively mitigate the performance risks posed to astronauts by chronically restricted sleep in spaceflight. The project will also provide estimates of the long-term effects of optimal sleep schedule countermeasures on hormonal profiles, sleep inertia and related physiological and neurobehavioral functions. Finally, the project is providing performance technologies and needed data for the development of a biomathematical model of human performance capability relative to sleep schedules and circadian physiology. These techniques will ultimately aid astronauts in the self-management of sleep and alertness during long-duration spaceflight.
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