ment of cruise (i.e., early in flight). Observers assessed the crew during normal flight operations using the “threat and error management model” (Helmreich, 2000; International Civil Aviation Organization, 2002; Klinect, 2002). In this model, safety was defined as the active process of crews’ effective management of operational threats, which included aspects of normal flight operations that have the potential to negatively affect safety, such as adverse weather or an aircraft system malfunction, and management of the inevitable errors that occur as part of normal human performance. The study found that restricted sleep in both the 24-hour and 48-hour periods prior to each sector was associated with changes in crews’ threat and error management performance. Restriction to less than 6 hours of sleep in the prior 24 hours was associated with degraded operational performance and increased error rates (Thomas and Ferguson, 2010). The authors concluded that their findings support prior sleep as a critical fatigue-related variable.
A study of 19 long-haul pilots also found that sleep in the prior 24 hours was a significant predictor of self-rated fatigue and the measured mean response speed of the psychomotor vigilance task after international flight sectors (Petrilli et al., 2006). These investigators concluded that in order to minimize the risk of fatigue, the sleep obtained by pilots should be taken into account in the development of flight and duty time regulations. This provocative suggestion may or may not be considered relative to flight and duty time regulations, but the scientific evidence on how commuting may contribute to fatigue because of inadequate sleep and prolonged wakefulness may suggest that there is a shared responsibility for mitigating fatigue between pilots and carriers.
Sleep is a physiological phenomenon that is defined by measuring brain waves, eye movements, muscle activity, and other physiological processes. As noted in Chapter 4, in order to acquire 6 hours physiological sleep time, an average healthy adult must spend approximately 7 hours in bed, as physiological sleep occurs for 85-95 percent of time in bed for healthy sleepers. Thus, when it is necessary to obtain at least 6 hours of physiological sleep, the time in bed would have to be approximately 7 hours. Moreover, although some people can function at normal levels for a night or two with 6 hours of physiological sleep, repeated days of 6 hours of sleep can result in cumulative fatigue and its attendant cognitive performance deficits in a significant portion of the population (Van Dongen et al., 2003a; Mollicone et al., 2007). Therefore, a requirement of 6 hours of physiological sleep (i.e., 7 hours of time in bed) prior to duty should be considered a bare minimum for alert functioning, and its adequacy for pilot alertness should be periodically evaluated.