Definition
The body's internal 24-hour cycle that regulates sleep, wakefulness, alertness, body temperature, hormone release, and other physiological functions. It is set largely by exposure to daylight and darkness, and disruptions to it (such as night flying, shift changes, or crossing time zones) can degrade alertness, judgment, and reaction time.
Plain English
Your body has a built-in daily clock that tells it when to be awake and when to sleep. When that clock gets thrown off, you don't perform as well -- even if you feel fine.
Context Anchor
Seen in aviation human factors, fatigue, night operations, long flights, and discussions of jet lag.
Derivation
From the Latin circa (around) and dies (day) -- literally 'about a day.' The name reflects that the cycle runs roughly, but not exactly, 24 hours.
Why Pilots Care
Disruptions from night flights, time zone changes, or irregular duty can reduce alertness and raise the risk of errors during critical phases of flight.
Analogy
It is like having a clock inside your body. If the outside schedule no longer matches that clock, your body may still act as if it is time to sleep.
Grounding Statement
A pilot flying through the night may feel a strong drop in alertness because the body’s normal daily rhythm is telling it that this is a sleep period.
Intuition Check
Circadian rhythm does not mean simply “how much sleep you got.” It means the daily timing pattern your body follows, which can affect alertness even when you are trying to stay awake.
Example Sentence 1
After flying a red-eye across three time zones, the captain knew his circadian rhythm would be working against him for the next couple of days.
Example Sentence 2
Regulators set flight duty limits partly to protect pilots whose circadian rhythm has been thrown off by crossing several time zones.