contributor to variability in that person’s performance is cognitive fatigue,” especially during night work (Folkard and Tucker, 2003; Miller 2005, 2010).

The human body has complex operational requirements, one of which is adequate rest. During rest periods, such as sleep, vital functions occur (Matthews et al., 2012). When the body is deprived of rest, specifically sleep, those functions do not occur, and the ability of the person to perform degrades predictably. The effect is noticeable in both cognitive and muscular performance. Sleep deprivation also causes substantial physiologic changes, such as increases in blood pressure (Conquest, 1991; Matthews et al., 2012; Chan et al., 1993).

The need for sleep is complicated by the reaction of the human body to light and dark. Sleep during periods of light is not as effective in operational restoration of the human mechanism as sleep during periods of darkness. Research has shown that job effectiveness, as measured in terms of the speed and accuracy of trained workers, is highest between 7 a.m. and 7 p.m. and lowest during the predawn hours (Fokard and Tucker, 2003; Office of Technology Assessment, 1991).

Physical fatigue in people is manifested in deteriorated dexterity, decreased eye–hand coordination, tremors, discomfort, and loss of strength and endurance. It is often not only how long a person works or how much rest and sleep he or she receives, but also the type of physical and mental workload that the person is subjected to while awake that determines whether fatigue is present (Chaffin et al., 2006).

System design can play an important part in improving or optimizing the performance of human elements in the overall system (Costa, 2001). Conversely, human performance can be degraded by taking advantage of perturbations to normal operational cycles (for example, attacking before dawn) (Gunzelman et al., 2012).

Some research in the various aspects of human fatigue is focusing on how human performance can be modified—improved or degraded—through fatigue-related elements of system design and performance (Matthews et al., 2012). Research tends to be focused on working situations in which altered sleep patterns are necessary, such as military movements or shift-work environments (Hull, 1990; Office of Technology Assessment, 1991; West et al., 2007; Samaha et al., 2007).

It is clear that fatigue-inducing situations degrade human performance. They can lead to dangerous situations, such as when a transport worker’s decision-making ability is degraded because of fatigue. Research has shown that fatigue has serious effects on the human brain (Reeves et al., 2006, 2007). But research results are mixed as to how to optimize human sleep needs in operational environments that demand 24-hour activity, such as in medical facilities, prisons, manufacturing plants, and telecommunications facilities (Costa, 2001). The optimization of length and staggering of shift assignments are being studied, but no clear picture of a “right” answer has emerged. Each environment has a different type of worker and different operational requirements, and this may explain why the research findings are mixed.

Some technologies can affect fatigue. For example, there has been a revolution in the science and technology of light-induced human performance modification. The use of spectrum-tuned light sources can wake people up or cause them to become drowsy. Examples are the recent identification of the melanopsin-pigmented retinal ganglion cell system, which is exquisitely sensitive to 460- to 480-nm blue light, and the use of blue-light filtering (in Canada) and bluelight treatment (in Europe) to affect human performance in 24/7 operations. The energy in that narrow window—6 percent of the total visual light spectrum—can duplicate the effects of the energy of the total light spectrum. These developments can be expected to influence the design of lighting systems, computer display screens, and eyewear. For example, Casper (at the University of Toronto) has shown that filtering out 460- to 480-nm light dramatically improves vigilance and performance in the circadian nadir (Rahman et al., 2011); it reverses the nocturnal dip in performance and provides users an enormous advantage in nighttime and early-morning operations.

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