Fatigue is the general term used to describe physical and/or mental weariness which extends beyond normal tiredness.

Physical fatigue concerns the inability to exert force with ones muscles to the degree that would be expected. It may be an overall tiredness of the whole body, or be confined to particular muscle groups. Physical fatigue most commonly results from physical exercise or loss of sleep. Physical fatigue often leads to mental fatigue.

Mental fatigue, which may include sleepiness, concerns a general decrease of attention and ability to perform complex, or even quite simple tasks with customary efficiency. Mental fatigue often results from loss or interruption of the normal sleep pattern and is therefore of great concern to pilots and ATCOs, who are frequently required to work early in the morning or at night.

Sleep patterns are naturally associated with the body's circadian rhythms. Shift patterns and transit across time zones can interrupt circadian rhythms so that, for example, it may be difficult for flight crew or pilots on duty in the early hours of the morning or flight crew operating long-haul routes through multiple time zones to achieve satisfactory rest prior to commencing duty.

It is important to note that people are not the best evaluators of their own alertness state. They are often sleepier than they report.

Fatigue Types

There are three types of fatigue: transient, cumulative, and circadian:

  • Transient fatigue is acute fatigue brought on by extreme sleep restriction or extended hours awake within 1 or 2 days.
  • Cumulative fatigue is fatigue brought on by repeated mild sleep restriction or extended hours awake across a series of days.
  • Circadian fatigue refers to the reduced performance during nighttime hours, particularly during an individual’s “window of circadian low” (WOCL) (typically between 2:00 a.m. and 05:59 a.m.).

Researches show that the accumulation of "sleep debt", e.g. by having an hour less of sleep for several consecutive days needs a series of days with more-than-usual sleep for a person to fully recover from cumulative fatigue.


Fatigue usually results in impaired standards of operation with increased likeliness of error. For example:

  • Increased reaction time;
  • Reduced attentiveness;
  • Impaired memory; and,
  • Withdrawn mood.

Typical Scenarios

In a pilot, fatigue may manifest itself by:

  • Inaccurate flying;
  • Missed radio calls;
  • Symptoms of equipment malfunctions being missed;
  • Routine tasks being performed inaccurately or even forgotten; and, in extreme cases,
  • Falling asleep - either a short "micro-sleep" or for a longer period.

In an air traffic controller, fatigue may result in:

  • Slow reaction to changing situation;
  • Failure to notice an impending confliction;
  • Forgetfulness.

Fatigue is also a signficant risk among aviation maintenance personnel due to nighttime work, rotating shift work,and long, unregulated duty times, among other factors.

Contributory Factors

  • Circadian adaptation, i.e. adjustment of the body internal clock (e.g. due to the shift pattern, jet lag, etc.)
  • Length of previous rest period;
  • Time on duty;
  • Time awake prior to duty (duties that start in the evening are more likely to cause fatigue than those beginning at e.g. 8 a.m.)
  • Sleep/nap opportunities (during the duty but also at layover destinations)
  • Physical conditions (temperature, airlessness, noise, comfort, etc.);
  • Workload (high or low);
  • Emotional stress (in family life or at work);
  • Lifestyle (including sleeping, eating, drinking and smoking habits) and fitness; and,
  • Health.



  • Ensure that work schedules, including consecutive shift-working patterns, are constructed so as to have the least possible impact on off duty - and, if applicable, on duty rest.
  • Seek to provide optimum working conditions;
  • Establish a Fatigue Risk Management System (FRMS), either as a part of the Safety Management System (SMS) or as a standalone system. An effective FRMS is data-driven and routinely collects and analyzes information and reports related to crew alertness as well as operational flight performance data. Computer models can be used to predict average performance capability from sleep/wake history and normal circadian rhythms.

Pilots and ATCOs

Adopt personal strategies which are likely to decrease the effects of fatigue such as the following:

  • Planning activities, meals, rest and sleep patterns during off-duty periods;
  • Making the most of permitted rest breaks, including naps;
  • Advising colleagues if one detects feeling drowsy;
  • Alerting colleagues if they appear to be becoming drowsy.



Related Articles

Accidents & Incidents

Events in the SKYbrary database which include fatigue as a contributory factor:

Further Reading







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