Factors Affecting Performance
Fitness, stress and the body clock
Fitness and health: general health, eyesight, hearing and freedom from incapacitating conditions underpin safe work; undeclared illness and self-medication are frequent contributory factors.
Stress follows the inverted-U (Yerkes-Dodson) curve: moderate arousal aids performance; too little (boredom, complacency) or too much (anxiety, fear) degrades it. Stressors may be physical (noise, heat), task-related (overload, monotony) or organisational (deadlines, rosters).
Time pressure and workload: deadlines erode checking; overload causes shortcuts and omissions, while underload causes loss of vigilance.
Shift work, circadian rhythm and fatigue: the body clock drives alertness peaks and troughs; night shifts, rotating rosters, early starts and long duty periods build sleep debt, which is cumulative and cannot be repaid by one long lie-in. Fatigue impairs reaction time, memory and judgement in ways the person cannot self-assess reliably.
Alcohol impairs judgement and coordination before the person feels affected, and is eliminated slowly (roughly one unit per hour, with wide individual variation), so morning-after effects are real. Regulations set limits and often a minimum bottle-to-throttle interval; the safe answer is none before duty. Medication and drugs: many common remedies (antihistamines, cough and cold preparations, some painkillers) cause drowsiness. Prescribed or over-the-counter drugs are compatible with duty only if the prescriber says so; caffeine and energy products mask fatigue rather than remove it.
Trap
Any option offering drink coffee to stay alert or sleep it off for an hour is usually wrong.
Rule of thumb: if you are too tired to drive home safely, you are too tired to certify an aircraft.