Fatigue, Sleep, Circadian Rhythms and Flight/Duty Time Limitations
What the domain tests
Fatigue questions require sleep physiology, circadian effects and flight/duty time rules, not just 'get more sleep'.
Core rules
- Sleep cycles about 90 minutes; NREM deep sleep restores physically, REM supports cognition. Fragmented or short sleep causes sleep debt.
- The circadian rhythm is driven by the suprachiasmatic nucleus and melatonin. The window of circadian low (WOCL) is roughly 02:00–06:00, with a secondary dip around 14:00–16:00.
- Microsleeps last seconds and are uncontrollable; sleep inertia after waking impairs performance.
- Signs: slowed reactions, fixation, poor scanning, irritability and forgotten checks.
- Countermeasures: strategic napping, controlled rest if permitted, caffeine, light, hydration and proper rest before duty.
- FTL rules limit flight time, duty and minimum rest; an FRMS manages fatigue risk beyond prescriptive limits.
Worked example
A pilot reports at 04:30 after 5 hours' sleep. The duty period starts inside the WOCL, when alertness is lowest. Even with 8 hours in bed the night before, the early report plus sleep debt raises risk. Mitigation: nap before duty, time caffeine early, and monitor carefully during the low window.
Traps
Do not say caffeine replaces sleep. Do not assume adaptation removes circadian effects. Do not treat one long sleep as full recovery from chronic debt. Do not confuse duty time with flight time.