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 hormone (Ball and Evans, 2001). The outcome, therefore, can range from performance errors to long-term health decrements.

Individuals who work at night and attempt to sleep during the day suffer because the timing of their sleep/wake schedule remains out of phase with the timing of the environmental light. Night workers are particularly prone to vehicle accidents, and their decreased alertness, performance, and vigilance are likely to blame for a higher rate of industrial accidents and quality control errors on the job, injuries, and a general decline in work productivity rate (Czeisler et al., 2001). Recent information also suggests that as the body normally releases melatonin when it is dark, working under artificial light at night suppresses the release of melatonin, which may increase the risk of developing cancer (Blask et al., 2002; Glickman et al., 2002, Blask et al., 2005; Stevens et al., 2007). In summation, ground-based evidence demonstrates that sleep loss, circadian desynchronization, and extended work shifts lead to increased performance errors and accidents. The extent to which these risk factors are also present in the space flight environment is therefore an important consideration.

Space flight research indicates that, overall, sleep quantity and quality in astronauts are markedly reduced in comparison to terrestrial sleep quantity. Seven Category II and Category III studies, which used polysomnographic measurements, actigraphy, or other measures, have consistently shown that astronauts sleep, on average, fewer than 6 hours per day (Table 3-1). This amount of sleep is between 1.5 to 2 hours fewer than the 8 hours that are recommended for astronauts per NASA-STD-3001, Vol. 1.

A post-flight debriefing survey that was conducted in 1988 (Category III) found that 58 crew members from nine space shuttle missions (ranging in duration from 4 to 9 days) reported sleeping on average 6 hours per day while in space compared to 7.9 hours terrestrially (Santy et al., 1988). Sleep was most reduced during the