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.
Plain English
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.
Context Anchor
Seen in discussions of fatigue and obstacles to keeping an accurate picture of what is happening during flight, especially with night flights, early morning lessons, long duty days, or time-zone changes.
Derivation
From Latin circa ("around") and dies ("day") — literally "around a day." The term reflects that the body's natural cycle is about, but not exactly, 24 hours, which is why it can be pushed out of alignment.
Why Pilots Care
Disruption produces fatigue that degrades alertness, judgment, and reaction time, directly raising the risk of errors and loss of situational awareness.
Analogy
It is like trying to fly while your body clock is still set to bedtime. The clock on the wall may say it is time to work, but your body may still be acting as if it is time to sleep.
Grounding Statement
A pilot who normally sleeps at midnight may have weaker attention during a 2 a.m. flight because the body is still in its usual low-alertness period.
Intuition Check
Do not treat circadian rhythm disruption as just “feeling tired.” It means the body’s internal daily clock is out of sync with the time the pilot is expected to be alert and performing.
Example Sentence 1
After three consecutive red-eye flights, the captain recognized signs of circadian rhythm disruption and requested a longer layover before the next leg.
Example Sentence 2
Cargo pilots on rotating shifts often manage circadian rhythm disruption by protecting consistent sleep periods between duty days.