Subjective sleep quality diminished in flight in both the Gundel et al. (1997) and Dijk et al. (2001) studies. Studies by Gundel et al. (1997) and Monk et al. (1998) also revealed decreases in SWS and electroencephalogram (EEG) slow wave activity (SWA), reflecting the decrements in the putative restorative component of sleep that are known as Process S (Borb�ly and Achermann, 1999). In contrast, ground-based studies of sleep restriction have revealed a rapid increase in EEG SWS and SWA (Brunner et al., 1990). This discrepancy suggests that not only is sleep quantity reduced during space flight, but also that the restorative component of sleep may be disrupted in space, which may further increase the likelihood that waking neurobehavioral performance deficits will occur (Bonnet et al., 2005).
Individual, Physiological, and Environmental Factors that Contribute to Sleep Loss and Fatigue During Space Flight
Various factors influence the extent to which individuals experience sleep loss and fatigue in space. Differences exist among subjects when experiencing the deleterious effects resulting from inadequate sleep. Some may need less sleep and/or be more resistant to the effects of sleep loss on brain functions. Laboratory and field studies have found this to be the case for 10% to 30% of individuals when sleep loss is mild to moderate (Van Dongen et al., 2004, 2005b; Caldwell et al., 2005). For the majority of astronauts, however, sleep loss and fatigue remain a relevant issue, and self-report of alertness has been shown to be inaccurate under conditions of sleep loss (see above), even in motivated and trained individuals.
The space flight environment affects this risk as well. For instance, recent data indicate that noise levels on the ISS, even during sleep periods, can average more than 70 dB, and that the recordings have "maxed out" at over 90 dB during scheduled sleep episodes (Goodman, 2003). For comparison, a circular saw creates noise levels from 91 to 99 dB. The degree to which noise and environmental disturbances impact sleep during space flight missions remains to be determined.
Recent Category III unpublished data (Barger and Czeisler, 2008) confirm the findings of previous assessments of sleep quantity and quality on orbit. In particular, these findings suggest that the amount and quality of in-flight sleep is reduced in comparison to terrestrial sleep behavior for multiple reasons. Data from 23 astronauts who completed 274 sleep logs on nine shuttle flights indicate that in 163 (59%) of these logs, sleep was recorded as having been disturbed on the previous night. The most frequent causes of sleep disturbance were voids; noise; physical discomfort; other crew member disturbances; and temperature. These physiological and environmental factors may interfere with achieving good sleep quality on either the shuttle or the ISS, thereby depriving crews of the full restoration afforded by sleep. An evidence-gathering effort is under way by BHP researchers to evaluate the impact of these individual, physiological, and environmental factors on sleep and fatigue, and to address several BHP gaps concerning the effects of work-rest schedules, environmental conditions, and flight rules and requirements on sleep, fatigue, and performance.
Occurrence of Circadian Desynchronization in Space Flight
A recent summary of findings from several short-duration evaluations shows that circadian desynchronization can and does affect at least some crew members in space, largely as a result of lighting conditions, scheduling constraints, or other aspects of the space flight environment (Mallis and DeRoshia, 2005).
Limited research is available on circadian rhythms in space. From the studies that have been conducted, there are inconsistencies as to the degree of circadian desynchronization experienced in flight (Mallis and DeRoshia, 2005). As an example, Gundel et al. (1997) assessed the circadian rhythms (using body temperature) of four astronauts over a period of 6 to 8 days during their stay on the Russian space station Mir. In comparison to baseline measures, these astronauts displayed a phase delay of more than 2 hours. The phase delay was attributed
