Plain English Meaning
Your body runs on a roughly 24-hour clock that tells you when to be awake and when to sleep. When flying schedules push against that clock — late nights, early starts, or flights across time zones — the clock gets thrown off, and you feel tired and dull-headed at the wrong times.
Full Definition
A disturbance to the body's internal 24-hour cycle that governs sleep, alertness, hormone release, body temperature, and other physiological functions, typically caused by night flying, shift changes, or rapid travel across time zones. The disruption produces fatigue, reduced alertness, slower reaction time, and impaired judgment even when total sleep hours appear adequate.