Fatigue, Sleep and Circadian Rhythms
Fatigue and sleep
Fatigue is a state of reduced physical or mental performance. Acute fatigue follows short sleep or long duty; chronic fatigue accumulates over days. Sleep has stages, including slow-wave sleep and REM sleep, both important for restoration. Sleep debt cannot be fully repaid in one short nap, and the circadian low (roughly early morning and mid-afternoon) increases sleepiness.
Circadian rhythms and countermeasures
The body clock is driven by light and regulates alertness, temperature and hormone release. Time-zone crossings and night duty disrupt it. Countermeasures include strategic napping, controlled rest on the flight deck where permitted, good sleep hygiene, light exposure and careful rostering. Caffeine helps briefly but does not replace sleep. Alcohol and some medications worsen sleep quality.
Traps in questions
Examiners test the difference between acute and chronic fatigue, and between fatigue and stress. They may ask which sleep stage matters most: both slow-wave and REM sleep are restorative, so avoid oversimplifying. Another trap is assuming a nap fully restores performance; it helps but does not erase sleep debt. Also remember that the early-morning circadian low is a high-risk period.
Worked example
A pilot reports for a 04:00 departure after two short nights. This combines acute sleep loss with the circadian low. Rule of thumb: if you feel sleepy on the flight deck, use permitted controlled rest, caffeine and cross-checking, and report fatigue through the safety system.