Chapter 3
Human Health and Performance Risks of Space Exploration Missions
Introduction
Sleep disorders plague a staggering number of individuals. The authors of the 2007 Institute of Medicine (IOM) report Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem state that as many as "60 to 70 million individuals chronically suffer from a disorder of sleep and wakefulness, hindering daily functioning and adversely affecting health and longevity... a wide range of deleterious long-term health consequences are associated with chronic, or cumulative, sleep loss. These consequences include: hypertension, diabetes, obesity, heart attack, stroke, and psychiatric disorders such as depression or severe anxiety."
In addition to the negative health outcomes that are cited above, another risk that can result from sleep loss is an increase in performance errors. Evidence shows that 24 hours without sleep, or less severe but more chronic sleep loss, can lead to daytime feelings of fatigue and increase performance errors on a variety of tasks that require attention, memory, cognitive and psychomotor speed, and executive functioning (Harrison and Horne, 1998; Durmer and Dinges, 2006; Banks and Dinges, 2007). Research indicates that astronauts, on average, sleep fewer than 6 hours per day (Dijk et al., 2001; Barger and Czeisler, 2008). Several authors of Earth-based sleep-doseresponse studies reveal that sleeping 6 hours or fewer per day results in cumulative cognitive performance deficits (Belenky et al., 2003; Van Dongen et al., 2003; Dinges et al., 2005; Mollicone et al., 2008). Moreover, there is a disconnect between subjective and objective measures of sleep loss under these conditions; e.g., individuals who are suffering from sleep deprivation or fatigue may not be able to accurately gauge their degree of impairment, and, therefore, will not take appropriate countermeasures to mitigate the impacts that can arise from these conditions.
Crews who are on orbit and the ground teams who support them face not only the likelihood of recurrent sleep loss but also the risk of circadian desynchronization. Circadian rhythms regulate subjective alertness, cognitive functions, and sleep propensity as well as core body temperature, hormone secretion (including melatonin), and the nocturnal secretion of growth hormone. A misalignment of circadian rhythms results in disturbed sleep and impaired performance and alertness (Ball and Evans, 2001, p.144; Van Dongen and Dinges, 2005). On Earth, shift workers often experience circadian misalignment, especially when they are working over night or rotating shifts; shift work schedules are associated with increased risk of accidents and injuries (Dinges, 1995; Czeisler et al., 2005; Barger et al., 2006). Recent evidence suggests that shift work, which includes exposure to light at night, suppresses the normal nocturnal production of melatonin by the pineal gland; this suppression over time may increase the risk of developing cancer among individuals who are working shifts (Blask et al., 2002; Glickman et al., 2002, Blask et al., 2005; Stevens et al., 2007).
Work overload also poses a risk to the behavioral health of space flight crews. NASA management currently sets limits, which are known as "Fitness for Duty Standards," for the planned number of hours in which astronauts are to complete tasks and events. The planned nominal number of work hours for space crews is 6.5 hours per day; it is recommended that crew members not exceed a 48-hour total work week. NASA researchers have found that maintaining nominal work hours and workload is especially important during critical operations. The NASA definition of a critical overload workload for a space flight crew is 10-hour work days that are undertaken for more than 3 days per week, or more than 60 hours per week (NASA STD-3001, Vol. 1). Not only is the duration of the workday important, but so, too, is the intensity of the workloads for space flight crews. Astronauts who have taken part in high-tempo missions, from the historic Apollo to the current space shuttle missions, have accomplished complex tasks in the most dangerous surroundings while enduring hours of intense concentration. Anecdotal reports from veteran astronauts (Scheuring et al., 2007) indicate that at times of high intensity, workload can result in mental and physical fatigue. Field studies from the medical and aviation industries show that increased and intense workloads, particularly in conjunction with disturbed sleep and fatigue, can lead to
