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Controlled Rest on the Flight Deck: A resource for operators  (2018)
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Fatigue is defined by the International Civil Aviation Organization (ICAO) (2015, page xiii) as:

A physiological state of reduced mental or physical performance capability resulting from sleep loss, extended wakefulness, circadian phase, and/or workload (mental and/or physical activity) that can impair
a person's alertness and ability to perform safety-related operational duties.

Circadian phase refers to the timing of the internal circadian body clock that synchronizes physiological systems to promote wakefulness during daytime, and sleepiness during nighttime. Therefore, fatigue can arise when attempting to maintain wakefulness at night, especially during the window of circadian low (WOCL) that is close to the nadir in body temperature, or as a result of insufficient sleep attempted during the day. The circadian clock is primarily entrained by light exposure. Traveling to different time zones can cause desynchrony between the internal circadian body clock and the external light environment. This desynchrony can cause jetlag symptoms that include fatigue.
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Controlled, Deck, Flight, operators, Rest
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Alexandria, VA: Flight Safety Foundation. November 2018, https://flightsafety.org/wp-content/uploads/2018/11/Controlled-Rest.pdf
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Curator: Phil So
NASA Official: Alonso Vera
Last Updated: August 15, 2019