Fatigue is a complex phenomenon that is different, if ever so slightly, from sleepiness (Shen et al., 2006). It is most commonly thought of as a continuous feeling of sleepiness and tiredness, and has been formally defined by Phillips (2015) as a: Suboptimal psychophysiological condition caused by exertion. The degree and dimensional character of the condition depends on the form, dynamics and context of exertion. The context of exertion is described by the value and meaning of performance to the individual; rest and sleep history; circadian effects; psychosocial factors spanning work and home life; individual traits; diet; health, fitness and other individual states; and environmental conditions. The fatigue condition results in changes in strategies or resource use such that original levels of mental processing or physical activity are maintained or reduced. (p. 53) © 2022, International Journal of Aviation, Aeronautics, and Aerospace.All Rights Reserved.