Sleep loss in aviation. Let’s review

On January 25, 2016, in darkness and hazy weather conditions, the flight crew of an ATR 72 lined up the aircraft on the runway edge lights along the right-hand side of runway 27L instead of the runway centerline lights of runway 27L.

This resulted in a misaligned take-off roll over the elevated runway edge lights along the right-hand side of runway 27L leading to damages to the runway edge lights and the aircraft itself.

A combination of environmental, operational, and human factors contributed to the sequence of events:

– Dark night operation

– Reduced visibility

– Runway and taxiway environment, including an extra tarmac width on runway 27L, the absence of runway shoulder markings, the absence of taxiway centerline lighting, and the use of a displaced threshold

– Flight crew divided attention unintentionally provoked by the before take-off procedures and checks

– Flight crew fatigue

The serious incident occurred in dark night and under instrument meteorological conditions (IMC).

Despite the fact that the actual flight and duty time of the flight crew did not violate any flight and duty time requirements, the Danish Investigation Authority considered the flight crew to be fatigued in the morning of the accident day. Because of fatigue, flight crew performance was impaired equivalent to more than 0.05% blood alcohol concentration impairing flight crew vigilance and reaction times and might have impaired the flight crew decision- making process and the flight crew night vision adaptation and visual acuity.

(See: Pilots fatigue lead to a Danish Air Transport ATR 72 serious incident)


Sleep loss and circadian disruption created by flight operations can degrade performance and alertness, we all know that. Scientific examination of these psychological/physiological parameters has established a direct relationship between their degradation and errors, incidents and accidents. We all know that, too. Nevertheless, despite that knowledge, fatigue remains an ever present danger in flight operations.

The operational demands of the aviation industry and the growth in global long-haul, regional, overnight, and short-haul operations will continue to increase the 24hour/7days requirement on flight crews. Therefore, shift work, night work, irregular work schedules, unpredictable work schedules, and time zone changes will continue to be commonplace components of the aviation industry. These factors pose known challenges to human physiology, and because they result in fatigue and performance impairment they pose a risk to safety.

Extensive data are available that clearly establish fatigue as a significant safety concern in all modes of transportation and in 24-hr shift work settings. The safety risk posed by human fatigue in transportation has been recognized and addressed over 100 years. Its 24/7 operational demands can easily lead to degradation or impairment in all aspects of human capability particularly cognitive performance including decision-making, attention, reaction time, learning, memory and communication skills. When individuals performing safety-critical functions are affected by fatigue, there is a high risk of fatalities, injuries and environmental hazards as a result of accidents or incidents.

Additionally, there are other associated costs of fatigue, such as the financial costs of reduced productivity and potential liability issues.


On 2006 a study at UCE Birmingham on the effect of fatigue on 162 short-haul pilots reported 75% of the pilots had experienced severe fatigue with 81% considering the fatigue to be worse than two years previously.

On 2012 the European Cockpit Association carried out a survey about self-assess of the level of fatigue experienced by pilots. More than 6.000 European pilots were surveyed between 2010 and 2012 and the results were published as Barometer on Pilot Fatigue. Some of the survey results are:

  • Over 50% of surveyed pilots reported having experienced fatigue as impairing their ability to perform well while on flight duty.
  • 4 out of 5 pilots reported having coped with fatigue while in the cockpit, according to polls carried out in Austria (85%), Sweden (89%), Germany (92%) and Denmark (93%).
  • In the UK (43%), Denmark (50%), Norway (53%) and Sweden (54%) the surveyed pilots reported falling asleep involuntarily in the cockpit while flying. In the UK, a third of the pilots said to have woken up finding their colleague sleeping as well. 65% of Dutch and French pilots stated they have trouble with “heavy eyelids” during flight.
  • 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.
  • More than 3 out of 5 pilots in Sweden (71%), Norway (79%) and Denmark (80-90%) acknowledge having already made mistakes due to fatigue, while in Germany it was 4 out of 5 pilots.

On August 2013 was published the study Prevalence of Fatigue in a Group of Airline Pilots based on a 2012 survey of 1500 commercial airline Portuguese pilots, commanders (captains) or first officers between the ages of 20 and 65 who were on active duty and had flown during the previous six months. The assessed their fatigue using the nine-item Fatigue Severity Scale. Some of the results of the study are:

  • More than 90 percent reported having made fatigue-related mistakes in the cockpit
  • 66% said that they had more than once been so tired that they should not have been at the controls
  • Despite they admitted having been fatigued while flying, 82% said they had never reported themselves as “unfit for flight as a result of accumulated fatigue,” and only 11 percent said they had done so only once
  • Pilots flying medium- and short-haul flights —typically less than six hours long with multiple legs — reported higher levels of fatigue than those who flying long-haul flights

The US National Sleep Foundation published in March 2012 a 1,087 transportation professionals poll, which included 220 pilots, interviewed about their sleep habits and work performance. Some of the results of the pilot survey are:

  • 23% admitted sleepiness has affected their job performance at least once a week
  • 20% said sleepiness had caused safety problems on the job
  • 37% of pilots reported that their current work schedule did not allow adequate time for sleep
  • 50% said they rarely or never got a good night’s sleep on work nights

Fatigue was identified as probable cause, contributing factor, or a finding in 20% of 182 major NTSB investigations completed between 1 January 2001 and 31 December 2012, ranging from 40% of highway investigations to 4% of marine investigations.

Human fatigue can have a role in accidents causation by producing performance impairment when the individual is awake, or by inducing microsleeps or producing an uncontrolled and unintentional full sleep episode.



Fatigue refers to a physiological state of reduced mental or physical performance capability in which there is a decreased capacity to perform cognitive tasks and an increased variability in performance. It is an enabler of poor judgment and decision-making, slowed reaction times, and loss of situational awareness and control. It degrades a person’s ability to stay awake, alert, and attentive to the demands of controlling a vehicle safely.


Fatigue increases the desire to obtain sleep. It is also associated with tiredness, weakness, lack of energy, lethargy, depression and lack of motivation. To make matters worse, fatigue actually impairs the ability to self-judge just how fatigued the person is.

Fatigue results from an imbalance between:

  • The physical and mental exertion of all waking activities (not only duty demands); and
  • Recovery from that exertion, which, except for recovery from muscle fatigue, requires sleep.

It is associated with sleep loss, extended wakefulness, high mental and/or physical workload, long unbroken periods of work -known as ‘time-on-task’ fatigue-, and circadian phase. In addition, there are acute influences such as monotony, boredom and environmental factors at the workplace (i.e. at the airport during long breaks or on the flight deck) that can interact with the factors cited above to precipitate the apparition of fatigue.

Sleep loss and extended wakefulness

Several factors are involved in the development of a loss of sleep and fatigue in aviation: less-than-optimal sleeping conditions, early reporting time, rotating and non-standard work shifts, night duty, the time on duty, long spans of wakefulness during the working day, the number of sectors, the number of time- zones crossed and the number of consecutive flying days.

There are other factors, like stress, disease, noise, vibrations/movement, or an upright sleep posture that can also influence sleep quantity and quality. Indeed, it should be emphasized that 8 hours of sleep may fail to provide restitution if it is highly fragmented as a result of these or other disturbing factors. This is particularly likely to be the case with sleep taken during the day when a range of environmental factors often contributes to sleep disturbance.

The main cause of problems associated with the unusual rest and activity patterns experienced by aircrew is the conflict with the biological clock which synchronizes our activity with that of the environment. This clock guides the circadian rhythm by activating the body, through increased metabolism during the day, and reduced levels at night, to promote recuperation. The result is an increased alertness during the day and lowered wakefulness and functional capability at night, being the lowest point of performance at around 05:00h (Window Of Circadian Low-WOCL). The reduction in performance at the lowest point during WOCL is severe and will markedly impair various aspects of performance including attention, reaction time and co-ordination. Humans simply are not designed to operate under the pressured 24/7 schedules that often define aviation operations, whether the operations are short-haul commercial flights, long-range transoceanic operations, or around-the-clock and shift work operations.


Because performance and alertness levels are largely influenced by the complex interaction between sleep and the 24-hour biological clock, the cited factors determine in a predictable way the background level of fatigue/alertness at any given moment.

The various factors that contribute to fatigue are individually capable of causing dangerous impairments in performance. Using a standardized performance test in, both, sleep loss and alcohol consumption conditions, investigators could provide a blood alcohol concentration metric to compare results from the sleep loss condition. Results demonstrated that after 17 hours of continuous wakefulness, cognitive psychomotor performance decreased to a level equivalent to a blood alcohol concentration of 0.05%. After 24 hours of continuous wakefulness, performance was approximately equal to a blood alcohol concentration of 0.10%.(Dawson D and Reid K. Fatigue, alcohol and performance impairment. Nature, 388:235, 1997.)

There is a discernible pattern of increased probability of an accident as duty time increases for commercial aircraft pilots. 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 beyond the 10th duty hour. 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 beyond the 13th duty hour. The greater the hours of duty time for pilots the greater probability of an accident.

The ability of aircrew to sustain levels of alertness during long flight duty periods has been the subject of many scientific investigations. There is an interaction between time of day, or circadian factors, on the one hand, and time since sleep and time on task on the other, with the result that duties at certain times of day are particularly susceptible to the effects of fatigue:

1. Night duty (flights ending during / extending through the Window of Circadian Low- WOCL): Night duty hours are especially vulnerable to severe fatigue. The longer the duty period extends the greater the pressure for sleep becomes, and there is strong evidence to show that many crews fall asleep, either voluntarily or involuntarily, on the flight deck. The longer the flight, the greater the risk that both pilots will fall asleep at the same time. Their ability to respond to an emergency situation, or to land the plane in adverse conditions, may also be severely impaired.

2. Duty during day time – early departures: Whereas long duty hours can be sustained during many day-time flights, specific problems are associated with early starts. Early start times are inevitably associated with a loss of sleep, the result of which is lower levels of alertness throughout the entire duty period. The effect is exacerbated over several successive early starts, due to an accumulating sleep deficit.

However, the most serious problems arise when the factor combine to provide even larger impairments. Night duty will involve work at the circadian low and an extended time awake. The two together will cause psychomotor impairment similar to that which is seen after the ingestion of enough alcohol to generate a blood alcohol level of 0.08% (which is significantly beyond the drink-drive limit in most European countries).

If the lack of sleep extends into the next day (comparable to the late morning after a 6-hour time-zone crossing without any post-flight sleep), there is a large impact on memory processes – with an impairment in the ability to respond to changed circumstances. The effects are further exacerbated in the event that prior sleep (i.e. before the flight/night duty) has also been disturbed.

Moreover, sleep taken after night duty will be truncated by 1-2 hours, because the circadian rise of metabolism in the morning will interfere with the sleep process. Sleep during the day on layover (before commencing another flight duty) is shallow, disrupted and shorter than during the WOCL. This has a detrimental effect on alertness during the subsequent flight duty.

Time-zone transitions

Flights across 4 or more time zones lead to the disturbance of sleep and circadian rhythms. If in addition, the flight is performed at night it magnifies performance decrements and increases the desire to sleep. As a result of these disturbances, more time is required to recuperate, both during layover and after return to base, before further flying duty is undertaken.

Night flights from west to east (North America to Europe – Europe to Asia) have an inherent augmented risk of fatigue. To counter fatigue, some pilots will try to nap before a night–time leg. While this can be helpful in some cases, it cannot prevent fatigue in all pilots. Moreover, it is not always possible to obtain an adequate amount of good quality sleep during the day and performance decrements will persist.

In addition, these types of flights are characterized by long periods of darkness with few operational demands while on cruise altitude, creating inherently soporific conditions. It is not until the flight approaches dawn that pilots experience reduced sleepiness as the daylight and circadian rhythms start to alleviate some of the fatigue. Nonetheless, the high workload requirements of approach and landing have to be borne at a time when there is a significant risk of pilot fatigue.

Most of these pilots fly a small number of night-time legs per month and revert to sleeping at night when not working. The circadian system of pilots who fly only a small number of night–time legs will not adapt to working at night, and these pilots are likely to display performance decrements during the night–time leg in spite of any countermeasures.

 Quality of In-Flight Sleep

Crewmembers’ sleep in onboard crew rest facilities is lighter and more fragmented than sleep on the ground. Sleep during flight deck naps is also lighter and more fragmented than would be predicted from laboratory studies.


Sleep debt

A sleep deficit of more than 2 hours has a discernible effect on performance capability. An acute reduction of sleep to below 6 hours will cause increased fatigue and reduced performance in most individuals. Sleep loss will also accumulate across time: 5 hours of sleep per day will be equivalent, in terms of fatigue and performance, to one full night of sleep loss after 5 days.

The effects of restricting sleep night after night accumulate so that people become progressively less alert and less functional day after day. This is sometimes described as accumulating a sleep debt. This is a common occurrence for crewmembers when minimum rest periods are scheduled for several days in a row.

The shorter the time allowed for sleep each night, the faster alertness and performance decline. Spending 7 hours in bed for 7 consecutive nights is not enough to prevent a progressive slowing down in reaction time. The decline is more rapid if spent only 5 hours in bed each night, and even more rapid for those who spent only 3 hours in bed each night. This is described as a dose-dependent effect of sleep restriction.

The pressure for sleep increases progressively across successive days of sleep restriction. Eventually, it becomes overwhelming and people begin falling asleep uncontrollably for brief periods, known as micro-sleeps. During a micro-sleep, the brain disengages from the environment (it stops processing visual information and sounds.

Full recovery of waking function after sleep restriction can take longer than two nights of recovery sleep (i.e., longer than it takes the non-REM/REM cycle to recover). Indeed, chronic sleep restriction may have effects on the brain that can affect alertness and performance days to weeks later.

For the first few days of severe sleep restriction (for example, 3 hours in bed), people are aware that they are getting progressively sleepier. However, after several days they no longer notice any difference in themselves, even although their alertness and performance continues to decline. In other words, as sleep restriction continues, people become increasingly unreliable at assessing their own functional status. This finding raises a question about the reliability of subjective ratings of fatigue and sleepiness as measures of a crewmember’s level of fatigue-related impairment

The sleep-restricted brain can stabilize at a lower level of functioning for long periods of time (days to weeks).


Recovery from sleep loss

Lost sleep is not recovered hour-for-hour, therefore, recovery sleep should be longer than usual. The restorative value of sleep quality (sleep quality) depends on going through unbroken non-REM/REM cycles, which suggests that both types of sleep are necessary and one is not more important than the other.

The more the non-REM/REM cycle is fragmented by waking up, or by arousals that move the brain to a lighter stage of sleep without actually waking up, the less restorative value sleep has in terms of how a person feel and function the next day.

Once out of duty and given the opportunity to sleep at leisure, on the first recovery night, there is more slow-wave sleep than usual. Indeed, there can be so much slow-wave sleep that there is not enough time to make up REM sleep. On the second recovery night, there is often more REM sleep than usual. Only by the third recovery night, the non-REM/REM cycle is usually back to normal.

To reduce crewmember fatigue requires reducing the exertion of waking activities and/or improving sleep.


Managing fatigue is a very complex task that must go far beyond flight/duty/rest time limitations. All the cited factors preclude a simple solution. There is no a simple and unique one-size-fits-all approach strategy that works for everybody.

A variety of regulatory actions to address fatigue-related safety risks has been implemented, reviewed and revised. A lot of information has been published, multiple training courses and strategies have been designed. However, fatigue continues to cause and contribute to transportation safety events.

Sometimes, because of global and national economic issues, civil aviation authorities may be compelled by commercial pressures to ease their national regulations with respect to the provisions of regulations of other nations. Loopholes could be exploited by the industry, within the different regulations, to promote their economic benefit.

Regulators must be aware that flight/duty/rest regulatory schemes loopholes when taken together, and applied in an environment of increasing competition, may encourage, or indeed compel, individual airlines to operate everywhere close to the defined limits by business considerations. In that case, it is likely that this would lead to a significant reduction in the safety of airline operations. There would be a clear competitive advantage for those airlines that operate as close as possible to the limits specified in the regulations, but there would be a significant increase in the risk of fatigue-related incidents and accidents if operators were permitted to operate to the limits.

The complexity and diversity of operational requirements demand a variety of approaches. Concept development should be initiated to move beyond current flight/duty/rest regulatory schemes and toward operational models that provide flexibility and maintain the safety margin.

Scientific evidence has remarked the vital importance of adequate sleep, not just rest, for restoring and maintaining all aspects of waking function and the importance of daily rhythms in the ability to perform mental and physical work, and in sleep propensity (the ability to fall asleep and stay asleep). Therefore it is critical that the core human requirement for sleep be managed effectively and operations should reflect the fact that the basic properties of the circadian clock directly affect an operator’s performance, productivity, and safety.

Some Mitigation Strategies that could be taken into account are:

  • Proper work/rest scheduling is of primary importance
  • Duty periods should not be as long as 12 hours when they surpass the window of circadian low- WOCL
  • If there is early starts the flight duty periods should be shorter than the allowed by the regulations
  • There shouldn’t be more than 2 consecutive night duties
  • There shouldn’t be more than 2 consecutive early starts
  • Strategic napping and strategic use of caffeine as alertness-enhancing compound must be considered
  • There is good evidence that in-flight sleep improves subsequent alertness and reaction speed and is a valuable mitigation strategy. The 40 minutes planned rest period provide time for preparation for the rest period, falling asleep, and sufficient length time for an approximately 26 to 30 minutes nap to occur. After the nap, a recovery period of 15-20 minutes allows time for sleep inertia to dissipate if present and to brief the rested pilot before reentering the operational loop. The amount of sleep obtained in cockpit nap does not affect subsequent layover sleep or circadian rest/activities patterns.
  • Rest is not the same as sleep. A rest period with reduced physical or mental activity does not produce the same effects of sleep. Only sleep can reverse the physiological sleepiness.
  • Layover rest periods should provide sufficient time to recover from the immediate effects of the previous flight and to obtain sufficient sleep prior to the next report
  • A sleep opportunity of 8 hours may not be sufficient after a night flight or a flight with a large (= 4 hours) time difference. Thus, even a minimum rest period of 10 hours in many cases may not provide sufficient time for recovery.
  • Schedules must be designed to allow periodic opportunities for recovery and periodically include an opportunity for at least two consecutive nights of unrestricted sleep, to enable crewmembers to recover from the effects of sleep loss. This does not equate to 48 hours off. For example, 48 hours off duty starting at 02:00 would only give most people the opportunity for one full night of unrestricted sleep. On the other hand, 40 hours off starting at 21:00 would give most people the opportunity for two full nights of unrestricted sleep.
  • Additional nights may be needed for recovery if a crewmembers’ circadian body clock is not already adapted to the local time zone.
  • Recovery opportunities (sleep opportunities) need to occur more frequently when daily sleep restriction is greater, because of the more rapid accumulation of fatigue.
  • Rest periods should include defined blocks of time (sleep opportunities) during which crewmembers are not contacted except in emergencies. These protected sleep opportunities need to be known to flight crews and all other relevant personnel. For example, calls from crew scheduling should not occur during a rest period as they can be extremely disruptive.
  • Operators should also develop procedures to protect crewmember sleep at layover and napping facilities. if a rest period occurs during the day at a layover hotel, the operator could make arrangements with the hotel to restrict access to the section of the hotel where crewmembers are trying to sleep (such as no children, crewmembers only) and instruct their staff to honor the necessary quiet periods (for example, no maintenance work or routine cleaning)
  • There are computer models that incorporate the schedule information and provide an instant evaluation of alertness levels on any given roster. It is possible to compare the effects on fatigue of different rosters and to estimate the effects of modifying the pattern of duty in different ways as well as estimate the probability that either a single pilot or both pilots will be asleep on the flight deck at any particular time
  • Good sleep hygiene by flight crew is essential. Therefore, pilots and other aviation personnel, particularly those performing overnight operations especially during the window of circadian low, must be deeply and recurrently trained about the physiology of sleep and circadian rhythm. The training should include the causes, effects and risks associated with fatigue as well as it’s prevention and mitigation strategies. Personal responsibility during non-work periods, rest environments, and the effects of commuting and/or napping should be stressed too. The fatigue training should include personnel involved in crew scheduling and senior management too.

Managing fatigue must take into account operational differences and differences among crew members and requires a comprehensive approach that focuses on research, education and training, technologies, treatment of sleep disorders, hours-of-service regulations, and on- and off-duty scheduling policies and practices.

Ultimately, fatigue-related accidents can be avoided with a combination of science-based regulations, comprehensive fatigue risk management programs, and individual responsibility.

To be continued…


  1. Paper prepared for the European Transportation Safety Council “Meeting to discuss the role of EU FTL legislation in reducing cumulative fatigue in civil aviation” in Brussels on Wednesday 19th February 2003. Akerstedt, R. Mollard, A. Samel, M. Simons, M. Spencer. European Committee for Aircrew Scheduling and Safety.
  2. Fatigue in transportation: NTSB investigations and safety recommendations. Jeffrey H Marcus, Mark R Rosekind. Marcus JH, Rosekind MR. Inj Prev Published Online First: February 29, 2016. doi:10.1136/injuryprev-2015-041791
  3. Prevalence of fatigue among commercial pilots. Jackson CA1, Earl L. Occupational Medicine (Lond). 2006 Jun;56(4):263-8
  4. Barometer on pilot fatigue European Cockpit Association. 2012
  6. Are pilots at risk of accidents due to fatigue?. Goode, J. H. Federal Aviation Administration, Office of Aviation Policy and Plans, Washington, DC 20591, USA, March 2003
  7. Pitch Excursion Air Canada Boeing 767–333, C–GHLQ North Atlantic Ocean, 55°00’N029°00’W 14 January 2011. Transportation Safety Board of Canada (TSB)
  9. FAA Advisory Circular AC No: 120-100. Subject: Basics of Aviation Fatigue. Federal Aviation Administration, June 7th, 2010.
  10. Crew factors in flight operations IX: Effects of planned cockpit rest on crew performance and alertness in long-haul operations. Rosekind, Mark R, Graeber, R. Curtis, Dinges, David F., Connell, Linda J., Rountree, Michael S, Spinweber, Cheryl L, Gillen, Kelly A
    Mark R. Rosekind, Ph.D.1, LCDR David F. Neri, Ph.D. 1,2, and David F. Dinges, Ph.D. 3
    1NASA Ames Research Center, 2United States Navy Medical Service Corps, 3University of Pennsylvania School of Medicine.
  12. NTSB Most wanted list 2017. Reduce Fatigue-Related Accidents. National Transportation Safety Board, January 2017.


  1.  Stress and lack of quality sleep, factors leading to serious incident
  2. Pilots fatigue lead to a Danish Air Transport ATR 72 serious incident


minime2By Laura Duque-Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@dralaurita. Human Factors information almost every day 

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