and from studies with animals. However, little or no readily accessible data are available on sensation and proprioception. Some data are available, however, from isolated and confining analog situations describing predictors of behavior during long-duration missions (Palinkas et al., 1998).
Several studies have been performed to assess the effects of microgravity, nonvertical positioning, and simulated gravitational environments (short-arm centrifuge) on the neurovestibular system. These studies have focused on the adaptation to—and the relationship between—the visual system and body position during travel in space (Cohen et al., 1995). Countermeasures have been designed to test the effects of the centrifuge apparatus or body tilt on postural stability and possibly orthostatic tolerance (Black et al., 1999). Disconjugate gaze occurs and persists for weeks after space travel (Markham and Diamond, 1999). Markham and Diamond discuss the use of medication (promethazine) for SMS, including its impact on alertness, the mechanism of delivery, and possible antidotes (amphetamines) for drowsiness. No results of controlled trials, dose-response trials, or comparative efficacy studies have been published. Recent NASA Neurolab studies, however, deemphasize the potential importance of overt neurovestibular disturbance during space travel.
Evidence is accumulating that sleep is disrupted during space travel and that the circadian rhythm is disrupted (Box 2–2). This may be mediated through the neurovestibular system. Data from Mir suggest that the duration of sleep is reduced, that sleep is not as deep, and that in other ways sleep may be physiologically different from the sleep experienced on Earth (Putcha et al., 1999). Possible explanations include the low levels of light in the spacecraft, changes in light-dark cycles, and the misalignment of work-rest shifts with light cycles.
Long-duration space travel may produce even more aberrant sleep disruptions and associated abnormalities. These can result from disruption of the hypothalamic-pituitary axis, with the resultant release of growth hormone, and changes in cortisol peaks and valleys. Disruptions of the circadian rhythm may result in abnormal stress responses, diminished performance because of fatigue, and mood and behavioral changes (Mullington et al., 1996). Although astronauts frequently use sleep medication, it has been