Fatigue is a major safety concern in many fields, but especially in transportation, because fatigue can result in disastrous accidents. Fatigue is considered an internal precondition for unsafe acts because it negatively affects the human operator's internal state. Research has generally focused on pilots, truck drivers, and shift workers.
Fatigue can be a symptom of a medical problem, but more commonly it is a normal physiological reaction to exertion, lack of sleep, boredom, changes to sleep-wake schedules (including jet lag), or stress.
Fatigue can be both physical and mental. Physical fatigue is the inability to continue functioning at the level of one's normal abilities; a person with physical fatigue cannot lift as heavy a box or walk as far as he could if not fatigued.
Mental fatigue, on the other hand, rather manifests in sleepiness or slowness. A person with mental fatigue may fall asleep, may react very slowly, or may be inattentive. With microsleeps, the person may be unaware that he was asleep.
The Federal Motor Carrier Safety Administration identifies three main factors in driver fatigue: Circadian rhythm effects, sleep deprivation, and cumulative fatigue effects, and industrial or "time-on-task" fatigue.
- Circadian rhythm effects describe the tendency for humans to experience a normal cycle in attentiveness and sleepiness through the 24-hour day. Those with a conventional sleep pattern (sleeping for seven or eight hours at night) experience periods of maximum fatigue in the early hours of the morning and a lesser period in the early afternoon. During the low points of this cycle, one experiences reduced attentiveness. During the high points, it is difficult to sleep soundly. The cycle is anchored in part by ambient lighting (darkness causes a person's body to release the hormone melatonin, which induces sleep), and by a person's imposed pattern of regular sleeping and waking times. The influence of the day-night cycle is never fully displaced (artificial lighting is not strong enough to inhibit the release of melatonin), and the performance of night shift workers usually suffers. Circadian rhythms are persistent, and can only be shifted by one to two hours forward or backward per day. Changing the starting time of a work shift by more than these amounts will reduce attentiveness, which is common after the first night shift following a "weekend" break during which conventional sleep times were followed. The effects of sleep deprivation vary substantially from person to person.
- Sleep deprivation and cumulative fatigue effects describe how individuals who fail to have an adequate period of sleep (7–8 hours in 24 hours) or who have been awake longer than the conventional 16–17 hours will suffer sleep deprivation. A sleep deficit accumulates with successive sleep-deprived days, and additional fatigue may be caused by breaking daily sleep into two shorter periods in place of a single unbroken period of sleep. A sleep deficit is not instantly reduced by one night's sleep; it may take two or three conventional sleep cycles for an individual to return to unimpaired performance.
- Industrial or "time-on-task" fatigue describes fatigue that is accumulated during the working period, and affects performance at different times during the shift. Performance declines the longer a person is engaged in a task, gradually during the first few hours and more steeply toward the end of a long period at work. Reduced performance has also been observed in the first hour of work as an individual adjusts to the working environment.
Human factors are currently the most common factor of aviation crashes. In 1999, the National Aeronautics and Space Administration, NASA, testified before the U.S. House of Representatives that pilot fatigue impacts aviation safety with "unknown magnitude". The report cited evidence of fatigue issues in areas including aviation operations, laboratory studies, high-fidelity simulations, and surveys. The report indicates that studies consistently show that fatigue is an on-going problem in aviation safety. Fatigue can result in pilot error, slowed responses, missed opportunities, and incorrect responses to emergency situations.
A November 2007 report by the National Transportation Safety Board indicates that air crew fatigue is a much larger, and more widespread, problem than previously reported. The report indicates that since 1993 there have been 10 major airline crashes caused by aircrew fatigue, resulting in 260 fatalities. Additionally, a voluntary anonymous reporting system known as ASAP, Aviation Safety Action Program, reveals widespread concern among aviation professionals about the safety implications of fatigue. The NTSB published that FAA's response to fatigue is unacceptable and listed the issue among its "Most Wanted" safety issues.
Between 2010 and 2012, more than 6.000 European pilots have been asked to self-assess the level of fatigue they are experiencing. These surveys revealed striking results. Well over 50% of the surveyed pilots experience fatigue as impairing their ability to perform well while on flight duty. The polls show that e.g. 92% of the pilots in Germany report they have felt too tired or unfit for duty while on flight deck at least once in the past three years.
Yet, fearing disciplinary actions or stigmatization by the employer or colleagues, 70-80% of fatigued pilots would not file a fatigue report or declare to be unfit to fly. Only 20-30% will report unfit for duty or file a report under such an occurrence.
Fatigue in aviation accidents
Safety experts estimate that pilot fatigue contributes to 15-20% of fatal aviation accidents caused by human error. They also establish that probability of a human factor accident increases with the time pilots are on duty, especially for duty periods of 13 hours and above (see following statements):
"It is estimated (e.g. by the NTSB) that fatigue contributes to 20-30% of transport accidents (i.e. air, sea, road, rail). Since, in commercial aviation operations, about 70% of fatal accidents are related to human error, it can be assumed that the risk of the fatigue of the operating crew contributes about 15-20% to the overall accident rate. The same view of fatigue as a major risk factor is shared by leading scientists in the area, as documented in several consensus statements." 
"For 10-12 hours of duty time the proportion of accident pilots with this length of duty period is 1.7 times as large as for all pilots. For pilots with 13 or more hours of duty, the proportion of accident pilot duty periods is over five and a half times as high. [...] 20% of human factor accidents occurred to pilots who had been on duty for 10 or more hours, but only 10% of pilot duty hours occurred during that time. Similarly, 5% of human factor accidents occurred to pilots who had been on duty for 13 or more hours, where only 1% of pilot duty hours occur during that time. There is a discernible pattern of increased probability of an accident the greater the hours of duty time for pilots." See. Federal Aviation Administration, Office of Aviation Policy and Plans, Washington, DC 20591, USA, March 2003.
Examples of pilot fatigue endangering the safety of passengers are the serious incident in Iceland (in 2007), the Colgan Air accident in the US in 2009 (50 people killed – see America Acts on Pilot Fatigue) and the Air India crash in 2010 (158 dead).
An undisclosed judicial inquiry report, obtained in 2013 by the French news magazine Le Point cites pilot fatigue as contributing factor to the Air France Flight 447 accident. The AF 447 crash happened on 1 June 2009 during a night flight from Rio de Janeiro to Paris-Charles de Gaulle and cost the lives of all 228 people on board of the aircraft. The accident had previously been investigated by the French Accident Investigation Body BEA, whose report does not differ much from the judicial inquiry report, except for the conclusions on fatigue.www.dead-tired.eu
Fatigue has also been quoted in an official accident investigation report, published on 1 March 2013. This report identifies pilot fatigue as a probable contributor to the Afriqiyah Airways Flight 771 accident in 2010. During a night flight from Johannesburg (South Africa) to Tripoli (Libya) the plane with 93 passengers and 11 crew members on board crashed during a go-around at Tripoli airport. Sixty-one Dutch nationals were among those killed. Only one passenger, 9-year-old Dutch boy, survived the crash.
In military aviation
In early 2007, the 201 Airlift Squadron of the District of Columbia Air National Guard (ANG), successfully integrated the Fatigue Avoidance Scheduling Tool FAST into its daily scheduling operations. This integration required the full-time attention of two pilot schedulers, but yielded valuable risk mitigation data that could be used by planners and leaders to predict and adjust critical times of fatigue in the flight schedule. In August 2007, the Air National Guard Aviation Safety Division, under the direction of Lt Col Edward Vaughan, funded a project to improve the user interface of FAST, permitting daily use by pilot schedulers and integration with automated flight scheduling software. This improved, user-responsive interface, known as Flyawake (FlyAwake.org), was conceived and managed by Captain Lynn Lee and developed by Macrosystems. The project cited empirical data collected in combat and non-combat aviation operations, and challenged the U.S. government's established policies regarding fatigue as a factor in degrading human performance.
Many countries regulate working hours for truck drivers to reduce accidents caused by driver fatigue. The number of hours spent driving has a strong correlation to the number of fatigue-related accidents. Another way to prevent those accidents is by using DADS (Driver Alertness Detection System. It is a cloud based service that monitors a driver’s state of alertness in real time. According to numerous studies, the risk of fatigue is also greatest between the hours of midnight and six in the morning, and increases with the total length of the driver's trip.
Fatigue among doctors is a recognized problem. It can impair performance, causing harm to patients. A study using anonymous surveys completed by junior doctors in New Zealand found that 30% of respondents scored as "excessively sleepy" on the Epworth Sleepiness Scale and 42% could recall a fatigue related clinical error in the past 6 months
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