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 environment. Slam shifting can result in sleep loss and fatigue for astronauts (Category III). Recent data from the JSC Missions Operations Directorate (MOD) (Korth et al., 2006) reveal that critical operations often require slam shifting. In 2,043 days of ISS operations (2000-2006), slam shifts occurred on 13% of these days, typically before and during critical operations (e.g., dockings/undockings, taxi spacecraft relocations, EVAs). Such schedule changes force critical mission operations to occur against the body's natural circadian rhythm and after sleep deprivation.

Slam shifting also affects the ground teams that are supporting the ISS during critical operations when these NASA teams often are working overnight. As described previously, people whose employment requires that they work overnight shifts must try to remain awake and alert to function well at times when their circadian rhythm and homeostatic drive are promoting sleep. Category IV evidence that is derived from flight surgeons indicates that crew members have said that "the shifting (circadian) was tougher on them than they thought it was going to be" (flight surgeon S Johnston, personal communication, 2007).

Occurrence of Work Overload During Space Flight Category III evidence reveals work overload occurring during some space flight missions, including those of the Skylab and Apollo Programs. The workload during the second Skylab mission steadily increased over 8 weeks, while crew members of the third Skylab mission reported that they quickly ran into difficulty due to work overload. The fast-paced schedule and workload of the mission had initially caused these crew members to consistently "feel" behind on tasks as well as demoralized. At the start of the 45th day of their 59-day mission, the crew members of Skylab 3 elected to have a sit-down, during which they refused to perform scheduled tasks. Mission Control personnel later acknowledged that the schedule had been such that it had not given the crew members adequate time in which to adjust to their environment (Cooper Jr., 1996). Category III evidence from the Apollo Program also reveals that some of the Apollo crews reported serious mental fatigue while they were performing lunar EVAs (Scheuring et al., 2007). Current shuttle missions to ISS are recognized for their high-tempo nature as crews perform complex, critical tasks. Of the 22 EVAs that were conducted during 2007, nine of these dangerous, and critical, endeavors lasted 7 or more hours.

Space Flight Performance Errors Due to Sleep Loss, Fatigue, Circadian Desynchronization, and Work Overload While evidence indicates that sleep loss, fatigue, circadian desynchronization, and work overload have occurred during space flight, it remains unclear whether these factors directly affect the performance of a crew in space flight. A limited number of space flight studies have evaluated performance for sleep and fatigue effects, and, of those studies, many of them have very few subjects. In the limited studies in which performance was shown to be affected, questions remain regarding whether sleep loss and fatigue were the root cause. It is also difficult to ascertain causality and relevance to future long-duration missions, when the data from these studies are largely derived from short-duration space flight studies (Table 3-2).

One of the first studies to evaluate cognitive in-flight performance was conducted by Benke et al. in 1993. This Category II evaluation assessed the performance of one cosmonaut in several cognitive tasks at three intervals during a 6-day mission on Mir. These tasks evaluated response time and accuracy. In-flight performance on the tasks was compared with pre- and post-flight performance. No significant decrements resulting from a short stay in space were found in this case study.