Fatigue, Sleep and Circadian Rhythm
What the exam tests
Fatigue, Sleep and Circadian Rhythm tests the physiology of sleep, the circadian clock, fatigue effects on performance and the countermeasures and regulations that manage risk. Expect questions on sleep debt, circadian lows, microsleeps and FRMS.
Core rules
- Sleep architecture cycles through N1, N2, N3 (slow-wave) and REM about every 90 minutes. Slow-wave sleep restores physically; REM supports memory and emotional processing. Fragmented sleep reduces restoration even if total hours look adequate.
- Circadian rhythm drives alertness, temperature and hormone release. The main low is around 0300-0500, with a secondary dip around 1500-1700; night duty places work in the low.
- Sleep debt accumulates with restricted sleep and impairs vigilance, reaction time and judgement before subjective sleepiness is obvious. Recovery sleep helps but may not fully repay a large debt.
- Microsleeps are brief involuntary lapses, often with head nods or blank staring; they are a red flag for uncontrolled sleepiness.
- Fatigue is a physiological state, distinct from tiredness; it degrades complex tasks, communication and monitoring.
- Countermeasures: strategic napping, caffeine timed before the low, light exposure, rest breaks, and FRMS/FTL limits. Alcohol and sedating drugs worsen sleep quality and are incompatible with duty.
Worked example
A pilot reports at 0500 after sleeping 4 hours because of a late evening. The flight is quiet and level. The pilot's eyelids droop and the altimeter drifts. This is sleep debt plus circadian low; the correct action is to announce fatigue, use a planned rest or split duties, and avoid relying on willpower.
Traps
- Assuming caffeine replaces sleep.
- Believing a long lie-in fully repays chronic debt.
- Treating fatigue as a moral weakness.
- Using alcohol as a sleep aid before duty.