“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.
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.
Obstructive Sleep Apnea17
One sleep disorder receiving particular attention in operational settings is sleep apnea. Sleep apnea patients often complain of excessive daytime sleepiness. Studies have demonstrated that individuals with sleep apnea have a two to seven times increased risk for car crashes, but data on its role in aviation mishaps is lacking. Mild to moderate sleep apnea can cause the performance decrement equivalent to that of a 0.05 to 0.08-blood alcohol content18.
Obstructive sleep apnea (OSA) results from repetitive partial or complete upper airway collapse upon inspiration, resulting in loud snoring, hypoxemia, and subsequently, brief arousals. It is typically progressive, especially with weight gain. As OSA becomes worse, oxyhemoglobin desaturations become more frequent, longer and lower. Hypertension may develop or worsen, especially in the morning.
The repetitive brief arousals caused by sleep apnea lead to the same effects caused by sleep deprivation. OSA patients typically complain of daytime fatigue, sleepiness, morning tiredness, concentration impairment, memory impairment, and irritability. Nocturia, gastroesophageal reflux and morning headaches are also common symptoms. The amount of sleep required to maintain function may begin to increase. As in sleep-deprived individuals, those having OSA may tend to deny or underestimate the extent of impairment caused by the illness.
Obstructive Sleep Apnea is caused by multiple airway anatomical and medical problems. Allergic rhinitis, nasal septal deformity, nasal polyps, maxillary hypoplasia, micrognathia, retrognathia, soft palate elongation, adenotonsillar hypertrophy, and other causes of airway reduction contribute to the development of OSA. All must be considered and addressed if the illness is to be effectively treated. This disorder should also be viewed as a chronic and relapsing illness, needing periodic reassessment. Before treatment is attempted, OSA should be verified and the severity quantified by polysomnography. There is wide variation in individual impairment caused by OSA, and the true severity of the illness is not predictable based on history alone. The patient’s report of improvement following treatment interventions is not reliable, due to partial treatment response, placebo effects and fear of loss of occupation or benefits. The Multiple Wakefulness Test, a modification of the Multiple Sleep
Latency Test, assesses the ability to stay awake. It is probably less useful than sleep testing during real and simulated driving and piloting tasks, but the lack of simulator testing availability and validated measures of fitness for duty makes the MWT the most widely used assessment for alertness.
Weight loss is helpful in obese OSA persons, but unfortunately the recidivism rate is high. Airway devices or surgical correction are usually needed to correct the disorder. The devices offer rapidity of treatment, reversibility, flexibility, low risk and low cost when compared to surgeries. Compliance problems, unpredictable effectiveness, and the possibility of relapse limit both surgical and non-surgical approaches.
Nasal Continuous Positive Airway Pressure (CPAP) is highly effective, however, compliance can be a problem. Nasal occlusion, drying, claustrophobia, discomfort, and rhinitis often interfere with its use. Oral appliances that protrude the mandible are effective in mild to moderate cases of sleep apnea, and can be used in most operational situations.
Nasal septal repair, turbinate reduction, and nasal polypectomy together or alone are important adjunctive surgical treatments, but are rarely curative when done without surgeries to enlarge the oropharynx and/or hypopharynx. Even uvulopalatoplasty (UPP), currently the most commonly performed procedure for OSA treatment, is successful much less than 50% of the time when strict criteria (respiratory disturbance index of <10) for cure are used. The effectiveness of UPP is lower in the obese or those with multiple airway anatomical problems.
Concomitantly performing a genial advancement can increase the chance of cure. Maxillomandibular advancement is usually performed after UPP has failed, but may be done as the primary procedure in some individuals with maxillofacial problems. Glossectomy, hyoid suspension, and hyoid anchoring may pose more risk of morbidity. In-office surgical procedures that minimally alter the airway are unlikely to help OSA. Radiofrequency Volumetric Tissue Reduction (RFVTR) applied to the tongue base has the potential of being an additional assist in the treatment of OSA, but still needs further investigation.
An uncommon neurological illness, narcolepsy most often begins in youth. It most cases, the sleepiness is extremely severe and pervasive. Afflicted individuals often report the inability to stay awake even in the most stimulating circumstances. Difficulty awakening in the morning is common. Sleep attacks, or irresistible urges to sleep, are characteristic and are often aborted or prevented by naps of less than 30 minutes. Narcoleptics often report dreaming during naps. Cataplexy, a sudden weakness of voluntary muscles lasting up to five minutes, is brought on by emotions such as laughter, anger or excitement.
There is no associated dizziness, or post-event fatigue. The weakness can be generalized or affect specific muscle groups. Sleep paralysis can occur in narcolepsy. The patient awakens fully, yet is unable to move for a few seconds or minutes. Hallucinations are common in the transition into and out of sleep. Cataplexy, sleep paralysis and hallucinations are not always present in narcoleptics. The symptoms are all manifestations of aberrant control of REM sleep related processes.
Only partial and temporary control of the sleepiness can be achieved with stimulants such as modafanil, pemoline, methylphenidate and amphetamines. Cataplexy, sleep paralysis and hypnagogic hallucinations are improved by antidepressants and gamma hydroxybutyrate (sodium oxybate), but these medications are prohibited for certain occupations.
Following an overnight polysomnogram to verify adequate sleep and absence of sleep pathologies that could cause sleepiness and REM sleep onsets during the nap trials, the diagnosis of narcolepsy is made by performing a multiple sleep latency test (MSLT). Two or more REM sleep onsets with evidence of pathological sleepiness are desirable to confirm the diagnosis.
The MSLT is a better measure of the ability to sleep than the ability to stay awake; therefore it should not be used as a test for ability to maintain alertness or vigilance. Genetic testing for narcolepsy is not diagnostic, but there is a genetic predisposition for the disorder. For most afflicted individuals, the cause of narcolepsy is now believed to be due to autoimmune-caused degeneration of CNS hypocretin-orexin cells, triggered by unknown environmental causes in susceptible individuals. Deficiency of hypocretin/orexin may be commercially measurable in the near future, adding another diagnostic tool for the evaluation of narcolepsy.
Chronic insomnia affects 10 to 30% of the population. Its prevalence and importance in the aviation population is less certain. Studies of performance in chronic insomniacs have given mixed results, but more recent studies suggest that performance impairments and increased accidents do occur as a result. Insomnia is a complaint that may have multiple causes.
Insomnia due to anxiety and depression, for example, has more bearing on flight operations than insomnia due to sleep state misperception, in which there is adequate sleep, but impaired ability to perceive that sleep has occurred. More senior flight personnel have more difficulty adapting to unfamiliar (hotel insomnia), uncomfortable, noisy, or bright sleeping accommodations. Nearly all insomniacs develop poor sleep habits and frustration, which may perpetuate the problem. Therefore, good sleep habits and other behavioral techniques are essential to the effective treatment of insomnia.
Good Sleep Habits
- Use the bed for sleep only—avoid watching TV, music, business, arguing in bed.
- Avoid looking at the time.
- Avoid alcohol, caffeine, and heavy meals 3 hrs before bed.
- Schedule a worry time, planning session, and wind-down time before getting into bed.
- Make lists of things to do the next day, but not in bedroom.
- Make the bedroom quiet, comfortable, dark and secure. Use white noise generators that make a fanlike noise if the environment is noisy. No radios, waterfalls, birds, etc. Minimize disruptions, e.g. pets.
- Get out of bed after lying awake for more than 20 minutes—do something boring or run through relaxation techniques.
- Avoid exercise and hot baths within 3 hours of bedtime.
- Exercise (aerobic) regularly, in the morning or afternoon.
- Keep a regular bedtime and get up time even on days off.
- Do not spend excessive amounts of time in bed, e.g., if you can sleep only 7 hours, spend no more than 7.5 hours in bed.
- Avoid excessive napping, which can interfere with the ability to sleep at night.
Restless Legs Syndrome21
In addition to psychological causes of insomnia, physical disorders also commonly contribute to insomnia. Restless legs syndrome (RLS) is an annoying but non-painful condition, which usually manifests itself in the evening hours, or with prolonged sitting or lying. It is an extremely common cause of insomnia, ranging from 6 to 13% of men and 10 to 17% of women. Involuntary jerks and twitches are frequently observed while awake, and usually there are rhythmic leg jerks occurring about every 20-40 seconds after sleep onset (periodic limb movements of sleep (PLMS).
The afflicted individual experiences an overwhelming need to move the legs, which gives temporary relief. Caffeine, alcohol, anticholinergic medicines, antihistamines, antidepressants, antipsychotics, diuretics, decongestants, and theophylline can aggravate the condition.
Iron deficiency, anemia, uremia, vitamin deficiencies and electrolyte abnormalities can aggravate RLS. Iron supplements have been reported helpful, but because of the risk of iron overload disorders it is recommended that ferritin and iron panels be obtained and followed. A number of drugs have been reported effective in small case studies. Clonazepam, dopamine agonists (levodopa, bromocriptine, pergolide, pramipexole) and narcotic analgesics have been demonstrated to be the most effective, but the habituating medications are contraindicated for flight duty.
Some widely held yet inaccurate assumptions about fatigue and circadian rhythms are problematic or dangerous when they are applied to aircrew scheduling practices. Dinges22 identified six myths that are particularly hazardous in aviation operations:
Myth 3. A high degree of training, combined with past experience with sleep deprivation and shift work, is the key to avoiding performance problems associated with fatigue from overwork and rotating duty schedules.
Myth 4. Increased payer rewards can overcome fatigue by increasing the motivation of sleepy crews
Myth 5. Determining how much rest or sleep employees will need to avoid performance problems is a straightforward matter when establishing new work/rest schedules
Myth 6. Some people believe that fatigue really shouldn’t be a concern because fatigued aircrews often safely complete trips without having accidents.
Dinges pointed out the imperative of dispelling these myths at the management and operational levels of any organization. Properly dealing with these issues could save lives by improving both safety and performance. Managers must make a concerted effort to educate themselves and their employees about the nature and effects of fatigue, support efforts to enhance an understanding of fatigue-related problems, and develop effective and feasible countermeasures.
Current and future job demands (long work hours and irregular work schedules) are likely to continue or possibly even worsen, therefore pilots, crews, and management must be prepared to effectively deal with fatigue to sustain productivity, safety, and personal well being. Unfortunately, this goal is made difficult because sufficient knowledge about the causes of fatigue, the impact of fatigue on performance in individuals, and the types of strategies that are effective for controlling fatigue in flight operations are lacking.
It has been suggested that breaks improve performance by allowing physiological recovery, reducing boredom, and increasing worker satisfaction. The Hawthorne experiments indicated that job performance and productivity improved when work breaks were introduced. Angus23 reported that breaks were beneficial for temporarily overcoming fatigue in sustained operations. On the other hand, activity breaks have been evaluated to determine their effectiveness in promoting alertness and performance during flight operations.
Twenty-eight pilots studied in a B747-400 simulator were involved a 6-hour uneventful night flight24. Pilots were provided either five regularly scheduled 7-minute breaks (Treatment) or a single, mid-flight break (Control), which involved getting out of their cockpit seat, walking, and interacting socially. Pilots in the Treatment condition showed improved subjective sleepiness ratings up to 25 minutes after the break compared to Controls. However, the objective performance tests did not show improved results for the Treatment vs. Control conditions and, therefore, did not parallel the subjective findings that the pilots reported feeling less sleepy. It is apparent that more work needs to be done in evaluating rest breaks during daytime, night time and sleep deprived conditions to assess the impact on flight operations.
Cockpit Rest Periods
Laboratory-based studies have demonstrated that naps can improve performance and alertness. NASA pilot reporting data have indicated that spontaneous, uncontrolled sleep episodes were occurring in flight. Therefore, NASA conducted a field study to examine the effectiveness of a planned cockpit rest period to improve pilot alertness and performance during actual flight operations25. Twenty-one B747-200 flight crew participated in the study involving a 12 day trans-Pacific trip schedule with eight flight segments of about nine hours each, with approximately 24 hours layover between flights. Group 1 was allowed a 40-minute planned in-flight cockpit nap opportunity during a single flight segment, one crewmember at a time on a predetermined rotation (Nap Group). Group 2 had a 40 minute control condition identified, but were not allowed to nap during that time (Control Group). Both groups had the same measures performed, including subjective sleep/wake diaries, physiological measures of brain and eye movement activity (i.e. EEG, EOG), and vigilance performance using a sustained attention task.
On average, using standard EEG criteria, the Group 1 (Nappers) fell asleep in 5.6 minutes and slept for 25.6 minutes. Overall, there was a 34% improvement in performance for Group 1compared to Group 2 (Controls) and a 54% improvement in physiological measures of alertness during the last 90 minutes of flight. Overall, the Control Group had 120 “microsleeps” of five seconds or longer during that time, including 22 during the final 30 minutes of flight that involves touchdown. The Nappers had a total of 34 micros1eeps, with none occurring during the final descent and landing phase of flight. Importantly, there were no differences in the subjective ratings of alertness between groups, though the objective performance and physiological alertness measures showed significant improvements in the Nappers.
Proper flight schedules
Because subjective complaints of sleepiness, fatigue-related performance problems, and physiological indices of impaired alertness have been shown to be particularly problematic for personnel who work (physiological) nights and early mornings. One of the most obvious countermeasures for fatigue is to minimize the amount of work occurring at these times26. Because this solution usually is not possible, every effort should be made to construct duty schedules that allow sufficient time for sleep between work periods, eliminate double shifts, shorten the length of flights, schedule work periods that ensure people have opportunities to sleep at the most conducive times of day (sleep occurs more easily during the late night/early morning and mid-afternoon), and develop fixed shifts. Rotating shifts, regardless of how they rotate (day-to-evening-to-night, or day-to-night-to-evening) are the most disruptive.
The most effective strategy for minimizing fatigue on the job is to ensure adequate sleep before the duty period. Research has shown that sleep restriction severely degrades performance in a dose-response manner (i.e., the less sleep, the greater the performance decrement)5,27. Although it has been suggested that five hours of sleep per night may be sufficient to maintain normal functioning, performance and alertness deteriorate after restricting sleep to six hours per night, and uncontrolled sleep attacks occur when only four hours of sleep per night are allowed. Although personnel may think they can adjust to chronic sleep restriction, no objective proof substantiates this belief.
Currently, at least seven hours of sleep per day appears to be the minimum required to sustain adequate alertness in most people. However, sleep needs vary widely, and individual requirements range from about four to ten hours. To determine how much sleep is necessary to maintain daily alertness, crewmembers should adjust the amount of sleep they receive gradually and adhere to any new schedule for about 1 week before deciding whether more nightly sleep is needed. The focus should be on how much sleep is necessary to remain awake in sedentary situations, such as watching television, reading, riding in a car, attending meetings, driving, and performing routine work, because these are the types of situations in which physiological sleepiness most likely becomes evident. Also, staying awake in the afternoons often is more difficult than at other times of the day, especially if chronic sleep restriction is a problem.
In situations in which sleep is disrupted frequently or missed altogether, scheduled naps can serve as performance maintenance or a recuperative function to attenuate fatigue until normal sleep is possible23,28. This countermeasure is the best for fatigue besides full, eight-hour periods of restful sleep. Short naps can be used to promote alertness during long work periods that allow very little time to sleep. Napping has been suggested as a viable fatigue countermeasure in several situations, including aviation. Although two-hour naps are clearly helpful in attenuating performance declines associated with continuous work without sleep, even short naps (5-20 minutes) have been found to enhance productivity and safety in sleep-deprived workers.
Scheduled napping of almost any duration offers a significant performance advantage, although its efficacy is affected by a variety of factors:
Creating a suitable nap environment is essential. A small, dark, comfortable place where noise and distractions are minimized is best; otherwise sleep masks, foam earplugs, or a fan can reduce environmental sources of sleep disruption. Naps should be taken at appropriate times in the 24-hour period because circadian rhythms affect the ability to go to sleep (on average, whenever possible, it is best to take nighttime naps between 1AM and 6 AM and daytime naps between 2PM and 5 PM). Naps should be as close as possible to the beginning of the sleep deprivation period because sleep debt should be avoided. Naps are most effective if they occur before or soon after the start of the sleep-deprivation period; later naps, although not optimal, can substantially recover performance and alertness that has been lost.
Naps should be as long as possible, but keep in mind that at least 15 to 20 minutes should be allowed for sleep inertia (post-sleep grogginess) to dissipate before returning to flight-related duties. Naps during circadian dips are easiest to initiate but the most difficult ones from which to awaken, and once awake, a person may take several minutes to attain adequate alertness. Naps of 40 minutes to two hours are recommended.
Stimulants and sedatives are currently used in U.S. military and foreign commercial operations. There may be a role for stimulants such as modafanil, pemoline, methylphenidate and amphetamines in defined settings. The same is true for short and intermediate acting sedatives. Even short-acting sedatives can impair next-day performance, however, and reasonable concerns exist about the effect of stimulants on sleep, emotions and performance.
Stimulants have both disadvantages and advantages related to effectiveness, availability, abuse potential, and side effects. Caffeine is widely available and clearly effective for attenuating sleepiness, especially in people who don’t normally consume large quantities29. It is easily obtained (from soft drinks, coffee, and tea), and is socially acceptable. However, it is unlikely that caffeine is sufficient to maintain the alertness of sleep-deprived people who ingest it frequently, and in sensitive individuals or overdoses, it can cause GI disturbance, nervousness, tremor, irritability, headaches, and tachycardia. Side-effects that typically occur with caffeine are also observed with other stimulants discussed below.
Pemoline is a less well studied controlled stimulant with a slower onset of action than that of other stimulants30. Also, in rare instances pemoline has caused hepatic necrosis, and has fallen out of favor for use in most clinical settings.
Modafinil is a new controlled stimulant that appears (at high doses) to produce alertness levels similar to those produced by amphetamines31. Because modafinil reportedly has a low abuse potential and few side effects at lower doses, it may be an effective compound for attenuating sleepiness in occupational settings. However, adequate studies must be performed before modafinil is considered feasible for aviator performance maintenance.
To date, sufficient data are not available from field settings and at normal doses may not confer a significant advantage over caffeine32.
Methylphenidate is an effective stimulant that, although available for many years, has not been extensively researched in aviation settings. Like the amphetamines it is considered to have a high abuse potential and is stringently regulated. Its effects on the cardiovascular system do not appear to be as problematic as found with the amphetamines, but more than with pemoline or modafinil.
Dextroamphetamine has been the most heavily researched stimulant on the market. It has been available since the 1950s and has been shown repeatedly to improve the alertness and performance of fatigued individuals both in laboratory and field settings33,34.
While it appears that several compounds can stave off the effects of fatigue, their chronic use is ill-advised because of a lack of data on their long-term feasibility in applied situations, the potential of drug tolerance that eventually could make the compounds ineffective, or their known side effects. Broad reliance on the prescription stimulants is not recommended because of the difficulty controlling administration in civil aviation operations. Indiscriminate use will lead to abuse and possibly addiction with all of these compounds. However, for the time being, U.S. private pilots and flight crews are prohibited from using medications discussed above. Therefore, other countermeasures must be considered.
Bright light and melatonin
For shift workers, a possible option is the use of bright lights or melatonin to resynchronize the body’s rhythms to different work/rest routines, especially to the night shift35,36. Theoretically, because most fatigue-related decrements in performance and alertness occur on the night shift, and many of these problems have been attributed to the fact that the body’s rhythms cannot effectively synchronize to night work, any strategy that can promote adaptation to night duty should improve nighttime performance.
The timing of light exposure exerts a major effect on circadian rhythms, and research has shown that bright light can produce large phase shifts when the exposure is timed in conjunction with a new sleep/wake cycle. In its most simplified fashion, this would involve exposing personnel changing from day shift to night shift to bright light for at least two hours in the evening (ideally, before work, beginning at approximately 8 PM) and exposing night workers who are transitioning to the day shift to bright light after about 9 AM. In addition, because melatonin levels exert an effect on the body’s clock, research has shown that proper timing of melatonin administration may also influence the phase of the circadian timing system.
The best time to administer melatonin is directly opposite to when bright light exposure should occur. Therefore, day crews attempting to adjust to the night shift should take melatonin around 9 AM (to promote daytime sleep), and night crews attempting to transition to the day shift should take melatonin at about 8 PM (to promote evening sleep). The intervention times suggested above are based on assumptions about the average timing of peaks and troughs in the body’s temperature and hormonal rhythms, and these are not consistent for everyone.
The only way to ensure the proper use of light and melatonin is to evaluate each individual’s circadian rhythms before employing such interventions. Also, it is crucial to minimize conflicting influences (eg, artificial bright light at night followed by sunlight exposure on the way home from the flight line in the morning) to facilitate the efficacy of these strategies. Although both techniques are theoretically suitable for countering fatigue in personnel who work rotating shifts, their application may not be feasible in most applied settings, particularly those that employ shift rotations or compound time zone transitions that occur within three days.
Evidence indicates that exercise may be an effective, if only temporary, method for increasing alertness and arousal37. Moderate physical exercise has been found to improve cognitive performance on a continuous attention task in non-sleep-deprived volunteers, and fairly stressful exercise enhances cognitive vigilance in sleep-deprived subjects. In addition, physiological indications of sleepiness were reduced in sleep-deprived helicopter pilots who engaged in repeated ten-minute bouts of moderately difficult physical activity38. However, it seems that even if exercise promotes alertness in the short term, the effects are relatively short-lived (lasting only 10-30 minutes).
Therefore, in situations where frequent exercise breaks are feasible, the use of this countermeasure may be appropriate; however, such a requirement no doubt will make this an unacceptable alternative in most aviation settings. Also, it should be noted that heavy exercise, although providing stimulation during the activity itself, may make sleepiness and/or fatigue more pronounced later in the work period.
Temperature and noise
Two common techniques sometimes used in an attempt to improve alertness during driving and other types of tasks are exposure to cold air and listening to the radio. Unfortunately, little evidence supports the efficacy of either of these strategies. Fagerstrom and Lisper39 reported that listening to a car radio slightly improved reaction times in extroverted or inexperienced drivers, but Reyner and Horne40 found that listening to audio tapes or radio programs had only marginal effects on the driving performance of partially sleep-deprived subjects.
Cold air applied to the faces of these drivers was similarly ineffective. Also, while the radio temporarily decreased ratings of self-reported sleepiness more than the cold air, the effect was not associated with improvements in physiological measures of alertness. The use of either strategy is not recommended. If used at all, they should be restricted to times when a small, temporary boost in alertness is necessary only to inhibit fatigue until a more effective countermeasure can be applied.
Military organizations historically have sought to reduce the impact of sustained work on personnel by ensuring high levels of physical fitness. Although it stands to reason that fit people will be able to withstand longer periods of physical labor, little evidence exists that the same applies to the performance of cognitive tasks. However, Harmii et al. found that female night workers’ subjective complaints of fatigue were reduced after objective improvements in physical fitness41,42. Unfortunately, this did not appear to translate into performance changes on a short-term memory task. These results were partially confirmed by Angus et al, who found that highly fit individuals were no better than those of average fitness in sustaining intense cognitive work during sleep deprivation23. Thus, physical training programs are unlikely to effectively minimize the effects of fatigue on mental tasks.
Increasing environmental stimulation through physical activity, rest breaks, and other techniques appears to exert beneficial effects on alertness; however, the most effective countermeasures for fatigue are those that minimize circadian disruptions and remedy sleep deprivation, i.e. proper work schedules, strategic naps, and sufficient sleep periods. Although other approaches are effective for minimizing on-the-job sleepiness, such as avoiding shift work and relying on pharmacological interventions, these are not feasible in most aviation settings.
Specific Recommendations for Aircrews, Groundcrews and Air Traffic Controllers
Applying the previously discussed fatigue-management principles to improve the alertness of pilots and crews requires minor modifications because of their unique work situations. However, these same basic integrated strategies are applicable across a variety of settings.
Adhere to a consistent sleep routine even on days off
This means that aviation personnel (especially those working nights) should avoid reverting to a daytime schedule during the weekends or on days off. Also, they should not sleep longer than usual on non-work days. Either of these behaviors can have a deleterious effect on performance because they upset the body’s biologic clock or lead to sleep difficulties or fatigue problems later on.
Get the best sleep possible in advance of any trip (or long work shift)
Family and friends should be kept informed about work schedules so they can avoid interrupting the sleep period. Crew members themselves should take several actions to maximize their sleep quality, such as making sure the bedroom is cool, dark, and comfortable; minimizing distracting noise by unplugging the telephone, disconnecting the doorbell, and using a fan or earplugs to cover sleep-disrupting sounds; and avoiding smoking, caffeinated drinks, and alcohol before bedtime because these can disturb sleep. Also, they should avoid medications that contain caffeine.
If possible, take a nap immediately before a work shift (especially a night shift)
One of the primary determinants of sleepiness is the amount of time that has elapsed between the duty period and the last sleep period. Naps lasting from 40 minutes to two or four hours are known to be helpful as long as enough time is allowed for sleep inertia to dissipate on awakening (15 to 20 minutes should be sufficient). As was mentioned earlier, some of the best times for naps are between 1 and 6 AM or 2 to 5 PM because sleep occurs more easily at those times.
Sedentary activities tend to make people sleepy, so efforts should be made to increase environmental stimulation. Although most of these strategies are only minimally effective, it might be helpful when possible to do brief shoulder, arm, or leg stretches; engage in conversation; and take breaks whenever the opportunity arises. However, remember that any of these techniques cannot be expected to help for more than a few minutes. So if sleepiness becomes a problem, the only really effective strategy is to take a nap (perhaps between flights).
When alertness begins to deteriorate, use caffeine for a temporary boost
A typical cup of coffee has about 100 mg of caffeine, and research has shown that 200mg doses can help reduce sleepiness temporarily. Tea and many soft drinks also contain caffeine but less than the amount found in coffee. Although caffeine is an effective stimulant, it is important to remember that the more someone normally uses caffeine, the less effective it will be. The alerting effects take a while to begin (30 minutes to an hour), and they may not last for more than an hour or two. Caffeine should not be considered a replacement for sleep. Naps are far more effective in the long run. Naps are defined as intentional sleep lasting less than half the length of the major sleep period.
Society has increasingly changed sleep amount expectations and work demand to 24/7 operations causing sleep disruption and reduction. Extensive research into fatigue by the DOT and DOD has yielded important information about the resulting desynchronoses and techniques to improve performance and safety during prolonged and/or nighttime flying and shiftwork. Basic principles to keep in mind are summarized below.
What Aerospace Personnel can do:
- Do not overwork or under-sleep before flying.
- Take naps before and at the beginning of flights at night to improve performance during and at the end.
- Get two nights of normal sleep before to improve performance before beginning night flying or continuous operations.
- Get two nights of normal sleep at the end of an operation to recover from the effects of sleep deprivation.
- When on night shift or flying across multiple time zones, take naps during the day, especially in the mid-afternoon sleepiness phase.
- Realize that naps are a stopgap approach to improve performance and safety for limited periods of time, not an indefinite substitute for long sleep periods during biological night.
- Attempt to anchor sleep when sleeping in a different time zone by getting some of the sleep during home base sleeping hours.
- Remember, the longer the nap, the better the improvement in performance.
- Also remember, the longer the nap, the longer it takes to awaken (more sleep inertia). At least 20 minutes should be allowed to awaken from a nap to allow dissipation of sleep inertia.
- When possible, engage in conversation, stretch and move about to improve alertness.
- Use caffeine to help maintain alertness, but keep in mind that it will disrupt sleep if used too close to your desired sleep time.
- Alcohol use may interfere with sleep quality and performance—avoid it when working shifts or in the days leading up to night operations.
- Learn and use relaxation techniques and sleep hygiene to assist napping and adjustment to new circadian schedules.
- Maintain a meal schedule with healthy and nutritional food to minimize gastrointestinal problems associated with night operations.
- Recognize the signs of sleepiness in yourself and your crew. Speak up when you or your crew need rest or sleep.
- If you have reached the point that you think you cannot cope adequately with changing schedules, consider reassignment or a different occupation.
- If you have a sleep disorder, seek treatment.
What Employers of Aerospace Personnel can do:
- Minimize night flights and rotating shifts; institute fixed shifts in place of rotating shifts.
- Keep duration of operations for any single operator to less than ten continuous hours.
- Tailor the length of duty to the strenuousness of the operation (e.g. air rescue, combat, rotary wing operations should have fewer hours on duty than transport operations).
- Provide sleeping environments as free from noise, light, temperature extremes, and interruptions as possible.
- Lying down and sleeping is more beneficial than trying to sleep chest elevated, therefore, provide sleeping arrangements for crews that allow the best sleeping position.
- Provide one or two nights off in a long series of night operations to help restore function.
- Allow at least 2 days between completed night flights and shift transitions in normal, non-urgent operations.
- Sanction and support napping before and during night and transmeridian flight operations, especially the longer and harder ones.
- Adjust the length of the naps allowed and the amount of time needed to get over sleep inertia based on the amount of sleep loss and circadian disruption incurred.
- Assess individual crewmembers’ ability to tolerate shiftwork and jetlag, particularly as they age. Try to fit duty schedule to ability to function.
- Identify sleep disorders in personnel and facilitate effective treatment.
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