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 during the tasks were revealed. As a result, the investigators proposed that the decrements may have been caused in part by the effects of accumulated fatigue.

Newman and Lathan (1999) conducted a Category II experiment on cosmonauts during space flight and did not find impairments in a memory-search performance task, although tracking disruptions were apparent. A performance monitoring study by Schiflett et al. (1996) included daily assessments of the different mental functions of three astronauts during a 13-day shuttle mission. Impairments and decrements were found in tracking performance, time-sharing efficiency, and memory-search performance in space. The researchers hypothesized that the impairment in memory-search performance in two of the three astronauts was not related to microgravity but, rather, was a side effect of decreased alertness and fatigue.

To further investigate the relationship between sleep and performance on orbit, Dijk et al. (2001) conducted an evaluation of the sleep, circadian rhythms, neurobehavioral performance, and light-dark cycles of five astronauts during two space shuttle flights, STS-90 (Neurolab) and STS-95. The researchers assessed neurobehavioral function and performance by administering several different tests, including the 10-minute PVT; a 4-minute, two-digit addition task; and a memory task. Subjective assessments of performance and effort were also recorded.

Analysis of variance revealed that across performance and mood variables, there was a consistent trend toward worse performance in flight than was noted before or after flight (Dijk et al. 2001). A detailed analysis of the time course of changes involving neurobehavioral measures, which was based on two measures that were derived from the PVT and the probed recall memory test, suggested that most of the study subjects exhibited a decline in performance during the last week before launch, a further decline in flight, and a slow recovery postflight. While the effect for the number of lapses in attention on the PVT and for the median reaction time was not significant, this lack of effect could be due to the small sample size.

Although findings were not significant, this continuously declining performance appearing in short-duration flight, and the trend of improvement in subjects post-flight correlated with the amount of REM sleep. On return to Earth, subjects experienced a marked increase in REM sleep, and their subjective sleep quality and neurobehavioral performance recovered.

In summation, performance data from space flight thus far reveal some effects on accuracy, response time, and recall tasks; however, the quality of the evidence for performance decrements occurring as a result of fatigue and sleep loss during space flight remains uncertain. To date, no systematic attempt has been made to measure the performance effects of fatigue, sleep loss, circadian desynchronization, and work overload during space flight, and it is unknown whether the effect of these factors on performance significantly impacts the completion of mission objectives. More evaluation is therefore needed to accurately characterize this risk in space, and to understand how sleep loss, fatigue, circadian desynchronization, and work overload in flight translate into performance decrements. Other questions of interest include: Even if performance decrements exist on cognitive assessments, do these indeed translate into potential operational errors? And are decrements, when they exist, related to sleep, fatigue, circadian, and workload issues, or are they instead related to other aspects of the space flight environment? Undoubtedly, thorough evaluation is needed to accurately characterize this risk in space. Testing with the 3-minute PVT, which will be conducted on ISS starting in 2009, will include a larger number of subjects and test sessions to evaluate cognitive performance over the course of longduration missions.

As noted previously, ground evidence strongly indicates that sleep loss, fatigue, circadian desynchronization, and work overload lead to performance decrements for some individuals. Evidence from space flight clearly 102