Night Aeromedical Factors and Fatigue
Circadian rhythm and the night window
The body clock drives a circadian low, typically 02:00–06:00, when core temperature, alertness and reaction time are at their worst. A night-rated pilot must plan around this, not fight it. Time awake, cumulative sleep debt, early starts and broken rest all compound the effect; 17 hours awake can impair performance comparably to a modest blood-alcohol level.
Hypoxia, CO and other physiological traps
Rod vision is the first function lost to hypoxia, so night flying effectively raises the altitude at which oxygen is needed. Use supplemental oxygen at night above about 5,000 ft cabin altitude where available. Carbon monoxide is odourless; suspect it with headache, nausea, drowsiness or unexplained deterioration, and treat by selecting fresh air, closing the heater and landing. Hyperventilation causes dizziness and tingling; hypoxia causes euphoria and poor judgement. Alcohol, sedating antihistamines, smoking and dehydration all degrade night vision and judgement.
Fatigue management
Use a fatigue risk management approach: protect sleep before a night duty, plan strategic rest, limit caffeine late in the duty, and avoid self-medicating to sleep. Recognise the difference between tiredness, which improves with rest, and fatigue, which does not.
Worked example
A pilot starts at 08:00 and is rostered to finish at 02:00 after four legs. At 23:30 he feels alert and accepts a fifth leg. Reasoning: the circadian low is approaching, duty has exceeded 15 hours, and performance will fall despite the subjective feeling. The correct decision is to decline the leg or ensure a rested crew member operates, and to submit a fatigue report.
Traps
Treating coffee as a fix; relying on willpower in the circadian low; ignoring mild symptoms of CO; flying after a sedating antihistamine; and assuming daytime rest is equivalent to night sleep.