Definition of sleep
Physiology of sleep
Circadian rhythm and circadian desynchronosis (including jet lag)
Definition of fatigue
Causes of fatigue
Predisposing and contributing factors (individual, environmental, operational)
Symptoms, signs, and performance effects of fatigue
Prevention and countermeasures
“I departed from a parallel taxiway instead of the runway. I never thought I could ever do such a stupid, unsafe maneuver. Leading factor: fatigue; Second: “That will never happen to me” attitude; Third: complacency; Fourth: weather; Fifth: I was in a hurry to get home. We had already been up more than 24 hours even though, technically, we were legal … This event has cautioned me about two things: 1) fatigue is insidious, and 2) I’m not as good as I thought I was, but I’ll get better-for sure.”
---Anonymous corporate pilot, quotation in Callback, NASA’s Aviation Safety Reporting System bulletin, Number 277, October 2002.
Since World War II, flight operations have been increasingly performed over longer distances, longer intervals, and across multiple time zones. The biology governing the performance of men and women has not changed, however. The timing, quality and quantity of sleep needed may vary among individuals, but among all people, unalterable physiological needs and constraints exist. Efforts are being made within aerospace to design and employ behavioral and pharmacological interventions to overcome the effects of fatigue and sleepiness in personnel required to operate in a sleep deprived condition and at times when they would normally be sleeping.
Complicating matters, however, is that ultra long haul aircraft that can fly 20 hours non-stop are in development, necessitating the need for more augmented or dual crews with appropriate sleeping compartments. The development and implementation of automated flight systems may result in new opportunities for crew resource management (CRM), allowing for fewer pilots to be in the cockpit and more time for alternate pilots to get adequate sleep. Unfortunately, the more automated systems may also promote complacency and inattention because there is not enough to keep the operators engaged with the aircraft.
Aviation accidents are caused by human error 80% of the time. The role of fatigue and circadian rhythm disorders (desynchronoses) in these mishaps is probably underestimated. The accident rate for long haul commercial flights is higher than for short and medium haul flights, leading to speculation that fatigue and sleepiness plays a more significant role with the larger transmeridian changes. It has been estimated that 15-20% of all transportation accidents are related to fatigue, which surpasses that of alcohol and drugs1.
Recognition of the causes and signs of fatigue is central to safe and effective air operations. Every flight operation has its own tempo, time required to perform the major tasks, personnel structure, and number of personnel. There are a number of different aerospace scenarios, ranging from ferrying operations to air rescue, combat and space flight. Prevailing cultural attitudes may pose a hindrance to adequate resting and napping. Our society now sleeps about an hour or two less on average than our ancestors a century ago. Sleep and the demand for productivity are at odds, and adult napping is virtually frowned upon.
The sleep culture of modern society has fundamentally changed and as a result, fatigue-related problems have reached significant proportions in the population. Technological changes have led to sleep requirements being pushed down on the needs scale. One-third of adults in a recent survey reported significant daytime sleepiness on the Epworth Sleepiness Scale, and 6% indicated they were severely sleepy2. Forty percent of adults indicated they were so sleepy during the day that it interfered with their daily activities, and 18% said they suffered from this type of problem several days a week. More than half the people surveyed also reported that chronic sleepiness adversely affected their mood, energy levels, concentration ability, and overall health, as well as their ability to pursue personal interests and maintain quality relationships with their family and friends. Obviously, daytime sleepiness exerts a negative impact on mental and physical well-being and general productivity. A
National Sleep Foundation (www.sleepfoundation.org) sponsored survey found that the U.S. workforce complains that on-the-job concentration, problem-solving, interpersonal relationships, and performance suffer because of fatigue. Both personal and on-the-job safety is adversely affected by sleepiness. Improperly managed pilot and air traffic controller fatigue can become a significant problem in flight environments that require alertness, complex judgment, and quick reactions. Fatigue touches every aspect of life in modern society, including aviation sectors in which requirements for unpredictable and extended work episodes often occur at times when alertness tends to be most compromised.
CAUSES AND EFFECTS OF SLEEP DEPRIVATION AND DESYNCHRONOSES
The Role of the Body Clock:
The internal circadian clock, located in the suprachiasmatic nucleus of the hypothalamus, is one of two principal physiological determinants of waking alertness and performance4. The circadian clock controls the 24-hour rhythm for a wide range of functions, including performance, alertness, behavior, and mood. One prominent circadian pattern is exhibited by the sleep/wake cycle, with biological programming for a consolidated period of daytime wakefulness and nighttime sleep, recurring in a regular 24-hour pattern. Alertness and the ability to perform are related to two basic neurophysiological forces: the body's circadian pacemaker (or biologic clock) and the drive or need for sleep (based on the length of previous wakefulness). Sleepiness cycles over a 24-hour period, and during this period, humans are programmed for two separate time frames of physiological sleepiness and two windows of alertness. For most, maximal sleepiness occurs at the lowest point of the circadian cycle, typically from about 3 to 5 AM, when the lowest levels in many functions are observed, such as temperature, mood, and performance. A second interval of sleepiness occurs at about 3 to 5 PM. The two windows of intrinsic alertness occur at approximately 9 to 11 AM and 9 to 11 PM.
Several factors affect the specific timing of these episodes of alertness and sleepiness and the degree of change observed during these times. The ability to adapt to a new time zone or shiftwork pattern takes up to 3 weeks, depending on individual differences, the frequency and magnitude of the time shifts. Environmental (light, activity) and social factors (sleep habits, social interactions, work schedule) may either assist or prevent the accommodation to a new schedule. Constantly changing shifts are more disruptive because people rarely remain on the schedule long enough to adjust.
Abruptly changing to a new schedule or time zone can result in both internal and external desynchronization. External desynchronization involves the internal clock being out of synch with external time cues. Internal desynchronization involves the internal rhythms (e.g., temperature, sleep/wake, hormone secretion) being out of synch with one another. It can take from a few days to weeks for full circadian resynchronization. The adjustment time needed is dependent on factors such as direction flown, number of time zones crossed, and light exposure.
Light is perhaps the most powerful cue that sets the circadian clock. Light exposure at times of clock sensitivity can be used to alter the clock and reset it to a new time zone or shift schedule. It can take 48 to 72 hours with expert application of light/dark cues to readjust the internal circadian clock. Therefore, there will not be significant circadian adjustment during trips of less than approximately 3 days; no matter what adaptation strategies are used.
The Role of Acute and Chronic Sleep Loss:
Human capability and performance can be reduced with sleep loss5. Studies have shown that decision-making, reaction time, memory, communication skills, mood, vigilance, alertness, and more can be degraded by sleep loss and circadian disruption. Operationally, it is important to note that these reductions may not occur as a smooth function. Performance becomes more variable when humans fatigue, and the onset of significant performance decrements can occur quickly. While performance may be at an acceptable and consistent level at one point, only moments later it can become irregular and significantly eroded. Falling asleep represents the absolute performance failure. However, performance can be reduced and can represent a safety risk well before or in the absence of an unplanned sleep episode.
Sleepiness and fatigue cause reduced ability to function. Chronic sleep restriction to fewer than 6 hours per night has been shown to impair performance and increase the tendency to involuntarily fall asleep during normal wakefulness. Besides the chronic sleep deprivation, it is without doubt that many transportation workers are at serious risk for sleepiness while operating because of circadian disruptions from rotating between day and night duty periods.
Schedule rotation is a problem for aviation personnel as well. Studies have shown that irregular work schedules, compared with consistent daytime work schedules, are associated with an increase in vehicle accidents, more frequent complaints about inadequate daily sleep, a greater reliance on caffeine to boost performance, and an increase in the probability that alcohol will be used as a sleep aid.
Lapses (the failure to respond to a situation) increase with increasing levels of fatigue. Lapses may be associated with microsleeps (episodes of sleep lasting 0.5 to 10 seconds), but can also occur without sleep onset. The four sleep-related factors involved in fatigue-induced performance impairments are the circadian phase of the biological clock, the presence of acute sleep loss, the presence of cumulative sleep loss and the presence of sleep inertia. Lapses increase 2 to 10 times during night operations without pre-existing sleep loss. Acute sleep loss (following a single night of sleep loss) results in 4 to 10 times more lapses, while chronic sleep deprivation by reducing sleep 2-3 hours per night for 1 week may increase lapses by 3 to 5 times normal.
Sleep inertia is the difficulty awakening from a sleep episode. Sleep inertia results in increased lapses and is most likely to be present after abrupt awakenings and awakening from stages 3 and 4 NREM sleep. The potential for catastrophe due to lapses is enormous. An aircraft going 250 kits on a glide path, for example, can travel over 400 feet during a 1-second lapse. Microsleeps have been shown to occur in aircrew during landing approaches in commercial carriers.
The degree of resulting fatigue and risk of mishaps are dependent on the type of aircraft, mission, operations schedule, and environmental conditions. Increased workload, noise, temperature extremes and turbulence tend to exacerbate the effects of sleep loss and jet lag. Reaction times may be markedly slowed, which can be critical when rapid reactions are necessary. False responding also increases, i.e. the pilot may take action when no action is warranted, especially when aware of having missed signals. The resulting anticipation of another event and over attention on individual signals or problems further reduces situational awareness. Fatigue increases calculation errors, logical errors and ineffective problem solving. The member is less able to think of new solutions and repeatedly tries the same approach to a situational problem.
Memory deficits progressively worsen with fatigue and sleep loss. The sleepy and tired crewmember reads or hears instructions repeatedly but cannot retain the information, leading to critical errors and uncertainty about the status of the situation. Performance variability results from increased lapses and errors of omission. Although the member often becomes aware of the shortcomings in performance and responds by trying to increase self-motivation and effort, performance improvement is short-lived. He/she may perceive the operation as more stressful and tiring as the effort continues. Ultimately, the crewmember’s motivation to perform well and avoid risks erodes.
No individual is immune to the effects of sleep loss and fatigue, although there are individual differences in the ability to tolerate sleep loss. After one night of sleep loss, half of healthy individuals perform reasonably well, but the remainder exhibit moderate to severe performance deficits. After 36 hours, there is little difference between individuals in their ability to perform---all have severe performance deficits.
The ability of a fatigued crewmember to self assess alertness is also limited6. In fatigued individuals, initial good performance early on may give a false sense of security. As time goes by, performance deteriorates. A crewmember is also more likely to overestimate his or her ability to perform if asked whether he or she is tired or able to perform. Relief from other crewmembers when signs of fatigue are observed (eyelids drooping, yawning, irritability, forgetfulness) is crucial.
Although fatigue once was thought to be a minor concern that could be overcome with sufficient motivation, training, or experience, it is now clear that the basis for this problem extends far beyond psychological factors.
Sleep is a physiological need similar to hunger and thirst and adequate undisturbed sleep is the only remedy for sleepiness. When aircrews try to ignore the sleepiness, they become even sleepier and must expend greater and greater amounts of energy to stay awake and complete tasks. Fatigue from long work hours, sleep deprivation, and circadian disruption has been recognized as a substantial cause of serious human errors, and most recently, the effects of fatigue have received heightened consideration in the aviation sector.
Research Studies in Real Aviation Settings
The NASA Fatigue/Jetlag Program was initiated in 1980 in response to a Congressional inquiry about whether fatigue was a safety issue in flight operations. In 1990, the program evolved into the NASA Fatigue
Countermeasures Program to emphasize strategies that would address the issue. Each branch of the Department of Defense and the FAA has also studied fatigue and its effects on safety and mission readiness.
The NASA Program used a broad range of research methodologies and measures. Research projects included studies in controlled laboratory situations, high-fidelity full-motion simulators, and field studies during actual flight operations7. The range of measures included subjective surveys and diaries, physiological measures of brain activity and core body temperature, and performance variables. A number of other scientists and groups have also conducted similar research.
To fully appreciate the impact of fatigue on overall safety, it is necessary to understand that many of our current scheduling and crew rest practices may be unintentionally contributing to degraded alertness. While societal demand and technology have evolved significantly, human physiology has remained unchanged. As humans, we have intrinsic physiological requirements for sleep and a stable internal biological clock. If sleep is lost or there is disruption of the internal clock, there are significant decrements on waking performance, alertness, and safety.
Humans were not designed to operate 24/7, which challenges the direction of modern society. The scientific data are clear that there are risks associated with presuming that human operators can function round-the-clock with the same efficiency, safety, and consistency now expected of our technology. Just as the machines have certain capabilities and operating limitations, so do the humans who design, implement, and operate them. The following are a few examples of data obtained from real air operations.
A NASA field study was conducted with 74 commercial pilots from two different airlines flying short-haul (flight legs less than 8 hrs) operations8. A sleep/wake diary was completed for three days before the trip, throughout the trip, and for three days after returning.
Core body temperature was monitored to examine circadian variables, and wake/rest activity patterns were assessed with a wrist worn movement device (autograph). A researcher accompanied the crews on the flight deck during the trips. Overall, the trips averaged 10.6 duty hours and involved 4.5 hours of flight time and 5.5 flight legs per trip day. About one-third of the duty periods were longer than 12 hours, and the average rest period (i.e., off-duty) was 12 hours long. The logbook data revealed that during the trip schedule, pilots reported that they slept less; awoke earlier; had more difficulty falling asleep; had lighter, less restful sleep; and poorer overall sleep quality. Overall, 67% of crewmembers had at least one hour less sleep per 24 hours during the trip (compared to pre-trip amounts), and 30% averaged two hours of sleep loss. Pilots reported consuming increased amounts of caffeine, snacks, and alcohol during the trip schedule. They also experienced more physical symptoms, including headaches, congested noses, and back pain.
Another field study involved 32 commercial pilots flying long-haul (> 8 hrs) trips that crossed up to eight time zones per 24 hours9. Overall, the average duty period was 9.8 hours long with an average layover of 24.8 hours. On two-thirds of the layovers, crewmembers slept twice, though they still averaged 49 minutes less than their pre-trip amount.
Circadian consequences of time zone crossings were evident in a variety of measures. Their circadian cycle moved to a 25.6 hr period, and about 20% of crewmembers had no discernable circadian temperature pattern. Overall, the circadian cycle did not synchronize to the continual time zone changes. There was more sleep loss associated with night flights compared to day flights. Pilots reported consuming more caffeine and snacks, but ate fewer meals during the trip schedule. Pilots had more somatic complaints, including headaches, congested noses, and back pain during trips. Logbook data and observer notes indicated that 11 % of flight crewmembers took a nap on the flight deck when conditions permitted.
A NASA study was conducted on 34 B-727 commercial pilots flying an 8-day overnight cargo schedule10. The schedules involved crossing no more than 8 time zones per 24 hours. Total sleep during trips averaged 1.2 hours less than pre-trip amounts, and the day sleeps were rated as poorer than nighttime sleep episodes. Altogether, 54% of crewmembers averaged more than one hour of sleep loss per 24 hours, and 29% of crewmembers lost more than two hours per 24 hours, across the 8-day schedule. All-night flight schedules did not result in a significant circadian shift (e.g., a shift toward alertness at night and sleep during the day), since there was only about a three-hour phase delay (i.e., instead of a circadian nadir at 4 AM, it moved to about 7 AM). Pilots consumed more snacks and experienced more physical symptoms, such as congested noses, headaches, and burning eyes during trips.
There was evidence of the circadian timing system observed, e.g. the length of morning sleep periods following an all-night duty period coincided with the underlying circadian wake-up time.
On some long-haul flights, extra crewmembers are available to allow pilot rest periods in onboard crew rest facilities on a rotating basis. Flight crew members (N=1404) completed a survey at three different commercial airlines flying several types of long-haul aircraft11. Altogether, flight crew reported taking about 39.4 minutes to fall asleep in the rest facility and averaging a total of 2.2 hours of sleep. The crews also rated 25 factors for whether they interfered or promoted good sleep in the crew bunk facility. The three factors that most promoted good sleep were physiological readiness for sleep, physical environment (e.g., bunk size, privacy), and personal comfort (e.g., blankets, pillows). There were five factors that most interfered with good sleep: environmental disturbance (e.g., background noise, turbulence), luminosity (e.g., lighting), personal disturbances (e.g., bathroom trips, random thoughts), environmental discomfort (e.g., low humidity, cold), and interpersonal disturbances (e.g., bunk partner).
Case Studies in Aviation Accidents:
The societal risks associated with human fatigue engendered by around the clock operations have been observed in a variety of ways. Fatigue has been identified as causal or contributory in multiple high-profile accidents. For example, official accident investigations have identified fatigue as causal or contributory in the Exxon Valdez grounding, Chernobyl nuclear accident, and the Space Shuttle Challenger.
In Guantanamo Bay, Cuba, in 1993, the crash of a DC-8 after a controlled flight into the terrain was attributed to "the impaired judgment, decision-making, and flying abilities of the captain and flight crew due to the effects of fatigue."
This occasion was the first time in history that the National Transportation Safety Board (NTSB) cited fatigue as a causal factor in a major U.S. aviation accident12. However, since then, there have been other cases with similar determinations. The NTSB ultimately ruled that a 1995 crash of another DC-8, which resulted in the deaths of three crew members, was in part a result of inadequate crew rest before the flight13. Authorities also found that fatigue contributed to the 1997 crash of Korean Air Flight 801, in which 228 people were killed when their Boeing 747 collided with a hillside five miles from the end of a runway in Guam14.
Additionally, after the 1999 Little Rock, Arkansas, crash of American Airlines Flight 1420, the NTSB found that aircrew fatigue was a contributing factor15. Eleven people died in this plane manned by pilots who reportedly had been on duty for more than 13 hours; the first officer had been awake more than 16 hours the day of the accident. Although a tragic event, one positive result from this crash has been a renewed interest in reviewing the adequacy of current flight-time limitations with some consideration toward developing new hours-of-service regulations for aviators. The flight and duty time rules currently on the books were revised in 1985, but the basic tenets originate in the 1940s.
Many factors can significantly affect sleep quantity and quality, but three deserve particular attention: age, alcohol, and sleep disorders. Dramatic changes in sleep occur as a normal function of the aging process. Stages 1 and 2 of non-Rapid Eye Movement Sleep (NREM) comprise over half of sleep after childhood and are the lightest stages of sleep. Stages 3 and 4 of NREM (also called deep NREM, delta or slow wave sleep) and rapid eye movement (REM) sleep comprise the remainder of the sleep and are considered the most restorative stages.
When approaching age 50 and older, five sleep-related changes typically occur: 1) There is a significant reduction in deep sleep, with some data suggesting that non-rapid-eye-movement (NREM) sleep stages 3 and 4 decreases or possibly disappears in the elderly. 2) There are more frequent awakenings during sleep, so that sleep is disrupted and quality is reduced. 3) Sleep becomes less consolidated, and it becomes increasingly difficult to obtain the same quantity and quality of sleep that was enjoyed when younger. 4) The ability to tolerate shiftwork declines, and, 5) Most sleep disorders increase in prevalence and severity with age.
Aggravating things, the most widely used sleep aid in America is alcohol, yet it can disrupt both that quantity and quality of sleep16. The specific amount of alcohol needed to disrupt sleep is dependent on a variety of factors, such as body mass and tolerance. However, the general effect is due to alcohol's potent suppression of the nervous system in the first half of sleep. As the alcohol is metabolized, rebound CNS excitability is seen and REM rebound occurs in the second half of the night, causing awakenings to occur more often. Nocturia is also increased. Therefore, alcohol consumed with the intention of promoting good sleep can instead reduce both the quantity and quality of sleep.
Yearly, at least a third of the adult U.S. population will complain about a sleep disturbance. There are a variety of physiological and psychological causes for these disorders. Often the sleeper is unaware of the disorder, and complaints may focus on disturbed nocturnal sleep or decreased waking function (e.g., sleepiness or performance problems). Many sleep disorders can be effectively diagnosed and treated at specialized sleep disorder centers by board-certified sleep experts.
Recognition and treatment of sleep disorders may lower the rate of aviation accidents and improve operational effectiveness.