Human Overload and Underload
Human Overload and Underload
This domain tests stress, fatigue, sleep and workload as measurable human limits. Questions ask you to identify the type of stress or fatigue, apply the inverted-U workload curve, or explain why an early report time is risky.
Stress and workload
- Stress is the non-specific response to demand. Acute stress is short-lived; chronic stress persists. Eustress improves performance; distress degrades it.
- Yerkes-Dodson: performance rises with arousal to an optimum, then falls. Complex tasks have a lower optimum, which is why high arousal hurts approach flying.
- Workload: too low causes vigilance decrements and boredom; too high causes tunneling, omission and error. Automation can shift workload rather than remove it.
Fatigue, sleep and circadian rhythms
- Acute fatigue follows one long duty; chronic fatigue accumulates over days; cumulative sleep debt cannot be repaid by one long lie-in.
- Sleep stages: NREM (including slow-wave sleep) and REM. Slow-wave sleep restores physical function; REM supports memory and mood.
- Circadian rhythm is driven by the suprachiasmatic nucleus; core temperature and alertness dip around 03:00-05:00 and 13:00-15:00. Melatonin rises in darkness.
- Shift work and early reports cut sleep and place duty at the circadian low. Countermeasures: strategic napping, light exposure, controlled caffeine, and fatigue risk management rather than willpower.
Worked example
A pilot reports at 04:30 for a four-sector day after sleeping from 23:00 to 03:45. The risk is not simply tiredness: sleep was shortened, the duty starts at the circadian low, and four sectors add cumulative fatigue. The reasoned mitigation is a pre-duty nap where possible, careful caffeine timing, and recognition that the second and third sectors may be worse than the first. Saying "I will just push through" is not a control.
Traps
Do not equate fatigue with sleepiness alone; fatigue also slows error detection and worsens mood. Do not say caffeine cures fatigue; it masks it and disrupts later sleep. Do not assume the body clock adapts to a night shift in one or two days. Do not describe underload as harmless; boredom produces missed cues. Do not treat crew rest as automatically restorative if the rest facility is noisy or the report time is early.