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Circadian challenge refers to the difficulty of operating in
opposition to an individual’s normal circadian rhythms or internal biological clock. This occurs
when the internal biological clock and the sleep/wake cycle do not match the local time. For
example, the sleep period is occurring at an adverse circadian phase when the body wants to be
awake. Engaging in activities that are opposite of this natural biological system represents the
circadian challenge (e.g., night work, shift work, jet lag).
source: Basics of Aviation Fatigue (FAA AC 120-100)
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