Definition
A degraded physical and mental state caused by insufficient sleep or prolonged wakefulness, which reduces alertness, slows reaction time, impairs judgment, and weakens vision — particularly night vision, where the eyes lose sensitivity and recover more slowly from glare.
Plain English
Being short on sleep, or worn out, so your body and brain don't work as well. You see less, think slower, and make worse decisions — and at night, your eyes struggle even more.
Context Anchor
Seen in night flying discussions, personal fitness checks, and go/no-go decisions before a flight.
Derivation
Deprivation' comes from the Latin 'privare,' meaning to take away — sleep deprivation is sleep that has been taken away from you. 'Fatigue' comes from the French 'fatiguer,' to tire. Together they describe a state where rest has been withheld and the body is paying the price.
Why Pilots Care
Fatigue is a leading contributor to night vision loss and pilot error; recognizing it early lets a pilot delay or cancel a flight before performance drops below safe limits.
Grounding Statement
A pilot who has been awake too long may feel capable, but small delays in seeing, thinking, and responding can become serious in flight.
Intuition Check
Do not assume fatigue means you are about to fall asleep. In aviation, fatigue can mean you are awake but still impaired enough to fly poorly.
Example Sentence 1
After a long day of work followed by a late evening flight, the pilot recognized the effects of sleep deprivation/fatigue and chose to delay departure until morning.
Example Sentence 2
Regulations require pilots to manage sleep deprivation/fatigue because it directly impairs the eye's ability to adapt to darkness.