Factors Affecting Performance
Health and fitness
Illness, injury, pain and both prescribed and over-the-counter medication all degrade performance; many common cold and allergy remedies cause drowsiness. Alcohol effects outlast the blood alcohol level: hangover reduces attention and judgement. Nothing - coffee, food, cold showers or fresh air - speeds up elimination. Caffeine improves short-term alertness but masks rather than cures fatigue, and it disrupts later sleep. Smoking impairs night vision and worsens the effect of altitude on oxygen uptake.
Stress
Acute stress narrows attention and encourages hurry; chronic stress steadily consumes capacity. Domestic and financial stress affect work performance just as much as task-related stress. A moderate amount (eustress) improves performance; beyond the optimum, performance falls sharply.
Workload, time and fatigue
Both underload (monotony, boredom) and overload (too many simultaneous tasks, constant interruption) raise error rates. Time pressure encourages shortcuts and skipped checks. Fatigue from long shifts, night work and insufficient sleep is the single largest contributor, and it is compounded by a mismatch between duty hours and circadian rhythm.
Traps
The idea that a determined person can simply try harder to overcome fatigue, or that a legally compliant duty roster guarantees fitness for duty. Also the assumption that anything sold without prescription is harmless.
Rule of thumb: if a question offers a remedy for fatigue, the only real remedy listed should be rest or sleep.