Sleep in Space Flight
Breath EasySleep Less?

David F. Dinges, Ph.D.

University of Pennsylvania, School of Medicine, Philadelphia, Pennsylvania

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Completion of the International Space Station is the most recent tangible evidence that the age of prolonged human exploration and habitation in space is dawning. There are, however, significant biomedical challenges to maintaining a human presence in space, and as more scientific research is conducted to mitigate the health risks posed by space flight, it is likely this work will provide insights into biomedical problems encountered on Earth. An excellent example of the latter outcome is the investigation by Elliott and colleagues in this issue of the American Journal of Respiratory and Critical Care Medicine (pp. 478-485) (1). They recorded respiration and sleep physiology in healthy astronauts during two National Aeronautics and Space Administration (NASA) space shuttle flights and compared these recordings to those made when subjects were Earth-bound before and after flight. They found that microgravity was associated with marked reductions in sleep-disordered breathing, in time spent snoring, in arousals during sleep, in respiratory rate during presleep waking, and in heart rate during both presleep waking and slow wave sleep. The results highlight not only the relative importance of gravity in ventilatory mechanics during sleep, but also reveal that within physically fit subjects there is a covariation between upper airway resistance, snoring, and the likelihood of respiratory- related arousals during sleep. It suggests Earth's gravity has a key role both in upper airway resistance and obstruction, and in the relationship of these factors to arousals during sleep.

The results of Elliott and colleagues (1) are novel, because other recent studies of human sleep physiology in space did not record respiratory variables (2, 3). These latter studies did, however, find altered and reduced sleep in space flight that in part motivated the hypothesis that sleep-disordered breathing might be the basis for the reports of sleep disturbances in microgravity. However the data of Elliott and coworkers (1) virtually rule out sleep-disordered breathing as a factor in space flight sleep alterations. In fact their results suggest that microgravity actually improves sleep by virtually eliminating arousals induced by upper airway resistance during sleep. Sleep improvement in microgravity is an unexpected finding in light of other evidence that sleep in space flight is disturbed relative to sleep on Earth.

Both the ability to sleep and the quality of sleep appear to be reduced in space flight, making hypnotic use in space relatively common45% of all medications taken by 219 astronauts on 79 shuttle missions were hypnotics for sleep disturbances, and they were used throughout the mission (4). Although current hypnotics are relatively safe and effective for sleep initiation, they cannot solve the more ubiquitous problem of reduced sleep duration in space flight. Despite NASA's recommendation that astronauts sleep 8 h each day while in space, reports over the past 15 yr have found that during space flight daily sleep durations average approximately 6 h (2, 3, 5, 6). The sleep results from Elliott and colleagues will be published elsewhere, but there are indications in their report that the sleep durations they recorded in space are in the range of 6.1 to 6.4 h/d (1), consistent with previous reports.

The cause or causes of reduced sleep duration in space flight remain occult. Candidates include circadian rhythm disturbance, environmental disruptions, motion sickness, excitement and stress, and mission work demands. It is likely these factors transiently affect sleep in space flight to varying degrees, but another possibility must also be considered. The marked reduction in sleep-disordered breathing engendered by microgravity may make sleep more consolidated, more efficient, and therefore afford a more rapid recovery of waking functions relative to the time invested in sleep. In other words, sleep duration may shorten in space flight because upper airway resistance is not fragmenting sleep and arousing the brain at the levels it normally does in Earth's gravity. Consolidation of sleep is essential for the homeostatic restoration of waking alertness and stable waking neurobehavioral functions (7). Fragmentation of sleep by frequent arousals, as occurs in moderate to severe sleep-disordered breathing, is typically associated with daytime sleepiness, elevated sleep propensity, and neurobehavioral deficits (8). Is it possible that even modest, nonpathological levels of arousal from sleep in healthy, physically fit subjects, can affect the recovery dynamics of sleep?

If subclinical levels of sleep-disordered breathing are reduced to near zero, as appears to be the case in microgravity, the physiological processes of sleep that result in waking restoration might occur more quickly and reduce the need for longer sleep duration. There is evidence that waking psychomotor vigilance capabilitywhich is highly sensitive to sleep quantityshows a saturating exponential function relative to sleep duration, such that the first few hours of sleep yield a much greater recovery of waking performance capability than subsequent hours of sleep (9). Perhaps reduction of arousals associated with respiratory events during sleep from an average of 5/h to 2/h, as Elliott and colleagues observed (1), can increase the recovery exponent as a function of sleep duration to the point where sleep time can shorten by 1-2 h while still providing full recovery. If so, astronauts chronically sleeping in microgravity for durations that average 6 h/d, should not experience the cumulative sleepiness and waking neurobehavioral impairments that have been recorded in Earth-based experiments on chronic sleep restriction (10). Although the astronauts appeared to be in need of sleep upon returning to Earth (1), the basis for their postflight fatigue is not known and may include the effects of returning to 1 g as well as any acute or cumulative sleep loss in space.

Clearly, to understand the implications of the findings of Elliott and colleagues (1) for health and safety during prolonged human habitation in space, the relationship between chronically reduced sleep in space flight and astronauts' waking neurobehavioral functions must be established. If we are to understand the implications of their findings for sleep on Earth, the mechanisms by which increased upper airway resistance affects sleep homeostasis must also be discovered.

	Footnotes

	References

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