Essay, Research Paper: Fatigue In Long Haul Pilot Operations

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Fatigue has been an issue in long-haul flight operations since the firsttransoceanic crossing by Charles Lindbergh. Today, modern aircraft havethe capability to fly farther, are more highly automated, and require fewerflight crewmembers for operation. While aircraft and the operational demandsof a global aviation industry have evolved, human requirements for sleephave not. Therefore, the physiological capabilities and limitations ofthe human operator remain central to maintaining the safety margin in long-haulflight operations. This paper will address some of the physiological mechanismsthat underlie fatigue, highlight findings regarding fatigue in long-hauloperations, and suggest some alertness management strategies.
Physiological Mechanisms that Underlie Fatigue
Since the mid-1950s, there has been extensive scientific research onsleep, sleepiness, circadian rhythms, sleep disorders, dreams, and the effectsof these factors on waking alertness and human performance (e.g., seerefs. 1 and 2). Some of the basic scientific findings regarding human sleep,sleepiness, and circadian rhythms that have emerged over the past fortyyears are critical to understanding the physiological mechanisms that underliefatigue in flight operations. Some of the significant information is presentedas a foundation for understanding the role of fatigue in long-haul operations(ref. 3).
Sleep is a Vital human Physiological Function. Historically,sleep has been viewed as a state when the human organism is turned off. Scientific findings have clearly established that sleep is a complex, activephysiological state that is essential to human survival. Like human requirementsfor food and water, sleep is a vital physiological need. When an individualis deprived of food and water, the brain provides specific signals-hungerand thirst-to drive the individual to meet these basic physiological needs. Similarly, when deprived of sleep, the physiological response is sleepiness. Sleepiness is the brain's signal to prompt an individual to obtain sleep;it is a signal that a specific physiological require-ment has not been met. Eventually, when deprived of sleep (acutely or chronically), the humanbrain can spontaneously, in an uncontrolled fashion, shift from wakefulnessto sleep, in order to meet its physiological need for sleep. The sleepierthe person, the more rapid and frequent are these intrusions of sleep intowakefulness. These spontaneous sleep episodes can be very short (i.e.,microsleeps lasting only seconds) or extended (i.e., lasting minutes). At the onset of sleep, an individual disengages perceptually from the externalenvironment, becoming unresponsive to outside information. Therefore, evena micro-sleep can be associated with a significant performance lapse whenan individual does not receive or respond to external information. Withsleep loss, these uncontrolled sleep episodes can occur while standing,operating machinery, and even in situations that would put an individualat risk, such as driving a car (refs. 4-6).
How much sleep does an individual need? An individual requires theamount of sleep necessary to achieve full alertness and the highest levelof functioning during waking hours. There is a range of individual sleepneeds and, though most adults will require about 8 hours of sleep, somepeople need 6 hours while others require 10 hours to feel wide awake andfunction at their peak level during wakefulness.
Sleepiness Affects Waking Performance, Vigilance, and Mood. Sleeploss creates sleepiness and often this sleepiness is dismissed as a minimalnuisance or as easily overcome. However, sleepiness can potentially degrademost aspects of human capability. Controlled laboratory experiments havedemonstrated decrements in most components of human performance, vigilance,and mood as a result of sleep loss. Sleepiness can be associated with decrementsin decision-making, vigilance, reaction time, memory, psychomotor coordination,and information processing. Research has demonstrated that with increasedsleepiness, individuals demonstrate degraded perform-ance despite increasedeffort, and report an indifference regarding the outcome of their performance. Individuals report fewer positive emotions, more negative emotions, andan overall worsened mood with sleep loss and sleepiness (for scientificreviews of this area, see refs. 7-10).
Generally, sleepiness can degrade most aspects of human waking performance,vigilance, and mood. In the most severe instances, an individual may experiencean uncontrolled sleep episode and obviously be unable to perform. However,in many other situations, while the individual may not actually fall asleep,the level of sleepiness can still significantly degrade human performance. For example, the individual may react slowly to information, may incorrectlyprocess the importance of the information, may find decision making difficult,may make poor decisions, or may have to check and recheck information oractivities due to memory difficulties. This performance degrada-tion canbe a direct result of sleep loss and the associated sleepiness and can playan insidious role in the occurrence of an operational incident or accident(refs. 11-13).
Sleep Loss Accumulates into a Sleep Debt. An individual who requires8 hours of sleep and obtains only 6 hours is essentially sleep deprivedby 2 hours. If the individual sleeps only 6 hours over four nights, thenthe 2 hours of sleep loss per night would accumulate into an 8-hour sleepdebt. Estimates suggest that in the United States today, most adults obtain1 to 1.5 hours less sleep per night than they actually need (ref. 14). During a regular work week this would translate into the accumulation ofa 5- to 7.5-hour sleep debt going into the weekend; hence, the common phenomenonof sleeping late on weekends to compensate for the sleep debt accumulatedduring the week. Generally, recuperation from a sleep debt involves obtainingdeeper sleep over two to three nights. Obtaining deeper sleep appears tobe a physiological priority over a significant increase in the total hoursof sleep. In other words, rather than sleeping 7.5 hours longer than normalon the weekend to "make-up" for the sleep debt accumulated duringthe week, the sleep-deprived person may sleep only slightly longer thannormal in a deeper sleep.
Physiological vs. Subjective Sleepiness. Sleepiness can be differentiatedinto two distinct components: physiological and subjective. Physiologicalsleepiness is the result of sleep loss: lose sleep, get sleepy. An accumulatedsleep debt will be accompanied by physiological sleepiness that will drivean individual to sleep in order to meet the individual's physiological need. Subjective sleepiness is an individual's introspective self-report regardingthe individual's level of sleepiness (refs. 4 and 15). An individual'ssubjective report of sleepiness can be affected by many factors, for example,caffeine, physical activity, and a particularly stimulating environment(e.g., an interesting conversation). However, an individual will typicallyreport being more alert because of these factors. These factors can maskor conceal an individual's level of physiological sleepiness. Therefore,the tendency will be for individuals to subjectively rate themselves asmore alert than they may be physiologically. This discrepancy between subjectivesleepiness and physiological sleepiness can be operationally significant. An individual might report a low level of sleepiness (i.e., high levelof alertness) but be carrying an accumulated sleep debt with a high levelof physiological sleepiness. This individual, in an environment strippedof factors that conceal the underlying physiological sleepiness, would besusceptible to the occurrence of spontaneous, uncontrolled sleep and theperformance decrements associated with sleep loss (refs. 16-18).
The Circadian Clock. Humans, like other living organisms, havea circadian (circa=around, dia=a day) clock in the brain that regulatesphysiological and behavioral functions on a 24-hour basis. In a 24-hourperiod this clock will regulate our sleep/wake pattern, body temperature,hormones, performance, mood, digestion, and many other human functions. For example, on a regular 24-hour schedule we are programmed for periodsof wakefulness and sleep, high and low body temperature, high and low digestiveactivity, increased and decreased performance capability, and so on. Anindividual's circadian clock might be programmed to sleep at midnight, awakenat 8 AM, maintain wakefulness during the day (with an afternoon sleepinessperiod), and then repeat the 24-hour pattern. The circadian rhythm of bodytemperature is programmed for the lowest temperature between 3 and 5 AMon a daily basis (ref. 19).
When the circadian clock is moved to a new work/rest (or sleep/wake)schedule or put in a new environmental time zone, it does not adjust immediately. This is the basis for the circadian disruption associated with jet lag. Once the circadian clock is moved to a new schedule or time zone, it canbegin to adjust and may take from several days up to several weeks to physiologicallyadapt to the new environmental time. Also, the body's internal physiologicalrhythms do not all adjust at the same rate, and therefore may be out ofsynch with each other for an extended period of time. Again, it can takefrom days to weeks for all of the internal rhythms to come together in asynchronous 24-hour rhythm in the new schedule or time zone.
There are some specific factors that can affect the circadian clock'sadaptation. Day/night reversal can confuse the clock so that the cues thathelp it adjust and maintain its usual physiological pattern are disrupted. Moving from a day to a night schedule and back to days can keep the clockin a continuous state of readjustment, depending on the time between schedulechanges. For example, severe effects would accompany a 12-hour day to nightto day schedule alteration. Another factor is crossing multiple time zones. While there is some flexibility for adjustment, putting the circadian clockin a time zone three or more hours off home time will require a reasonableamount of physiological adaptation. Another factor can be the directionthe clock is moved. Shortening the period (e.g., moving to a 21-hour cycleor day) is generally more difficult than lengthen-ing the period (e.g.,moving to 25 or more hours), which is the natural rhythm of the circadianclock. Therefore, it can be more difficult to cross time zones in an eastwarddirection compared to westward movement. It can also be more difficultto move a work/rest schedule backwards over the 24-hour day compared tomoving it forward (e.g., forward from day to swing to night shift). Allof the associated difficulties of moving the clock, such as poor sleep,sleepiness, effects on performance, and so on, will be affected until thecircadian clock physiologically adapts to the new schedule or time zone(refs. 20 and 21).
Scientific studies have revealed that there are two periods of maximalsleepiness during a usual 24-hour day. One occurs at night roughly between3 and 5 AM, and the other in midday roughly between 3 and 5 PM. However,performance and alertness can be affected throughout a 12 AM to 8 AM window. Individuals on a regular day/night schedule will typically sleep throughthe 3-5 AM window of sleepiness. The afternoon sleepiness period can bemasked by factors described previously, or present a window when individualsare particularly vulnerable to the effects of sleepiness. This also meansthat individuals working through the night are maintaining wakefulness from3-5 AM when their circadian clock is programmed for sleep. Conversely,individuals sleeping during the day are attempting to sleep when the circadianclock is programmed for wakefulness. However, individuals searching forspecific windows when they are physiologically prepared to sleep, eitherfor an extended sleep period or a strategic nap, can use these periods totheir advantage (ref. 4).
Interaction Between Sleep and Circadian Processes. At any giventime, an individual's propensity to sleep or, conversely, the ability tomaintain alertness and vigilance, will be the result of an interaction betweensleep and circadian processes. An individual's ability to fall asleep quicklyand obtain a good quantity and quality of sleep can be related to the prioramount of sleep (i.e., sleep debt) and circadian time of day. An individualwith no sleep debt attempting to sleep at a time of circadian wakefulnessand alertness will have difficulty falling and staying asleep. However,an individual with a sleep debt attempting to sleep at a time of maximalcircadian sleepiness will fall asleep quickly and easily maintain sleep. Also, an individual with a substantial sleep debt may be physiologicallysleepy enough to override circadian factors and be able to fall asleep ata circadian time for wakefulness.
These two factors also interact to determine an individual's level ofphysiological alertness and performance during waking hours. A third factorcan also be a consideration: the number of hours of continuous wakefulness. An individual with a sleep debt, awake continuously for 20 hours, and workingthrough the 3-5 AM circadian period of maximal sleepiness will have difficultymaintaining alertness and performance. However, an individual who has obtainedthe required amount of sleep, has been awake for 10 hours, and is workingthrough the 3-5 AM circadian low will probably have less difficulty maintainingwakefulness. Any one of these three factors can increase an individual'svulnerability for a performance decrement. Two or three of the factorscoinciding will increase the probability of a fatigue-related performanceproblem.
Individual Differences. It is critical to note that there aretremendous individual differences in these physiological factors. Thereis a range of sleep needs, differences in physiological flexibility foradaptation of the circadian clock, and ability to tolerate sleep loss orcircadian disruption. Therefore, while these fundamental properties ofsleep and circadian processes are factors for all human physiology, thereis a range of individual responses for any particular set of circumstancesor operational demands.

Alertness Management Strategies
There is no quick fix or magic bullet to address all of fatigue engenderedby long-haul flight operations. Unfortunately, there is no simple solutionthat will address all individuals, all operational demands, and all thetechnology currently involved in the aviation industry. Aviation requires24-hour operations, and a challenge facing the industry is how to incorporatethe scientific and physiological knowledge that currently exists into areasthat will maintain the safety margin. Therefore, every arena where theknowledge can be applied should be examined for potential improvements.
Four general categories for examination include hours of service, scheduling,design and technology, and personal strategies. Hours of service are affectedby both federal regulatory policies and contractual agreements. Schedulingis dictated by a complex variety of factors that are often idiosyncraticto a particular airline's operation. The automation evolution has broughttremendous advances to aviation, though its effects in a variety of domainsremain unclear. There is also a variety of personal strategies that canbe used to apply the current state of knowledge on a daily basis for flightcrews. Each one of these areas should be examined for mechanisms by whichto incorporate scientific and physiological information about fatigue. The challenge is to minimize the adverse effects of any particular categoryand, wherever possible, use each one to maximize alertness and performanceduring flight operations.
Personal Strategies. We have proposed differentiating alertnessmanagement strategies into two components: preventive strategies and operationalstrategies (ref. 29). Preventive strategies are used prior to duty or onlayover to minimize the adverse effects of the underlying physiologicalfactors (i.e., sleep loss and circadian disruption). These strategies includeobtaining maximal quantity and quality of sleep prior to duty, schedulingsleep periods during layover, accounting for fatigue factors during tripscheduling, napping, maintaining good sleep habits, exercising, maintainingbalanced nutrition, and others. Operational countermeasures are used inflight to maintain alertness and performance during operations. Generally,these strategies may be more short-acting and serve to mask or conceal underlyingphysiological sleepiness. These counter-measures include physical activity,strategic caffeine use, and social interactions.
As previously described, the only mechanism to reverse a physiologicalsleep need is sleep. With sleep loss, the brain will signal its need toobtain sleep (i.e., sleepiness) and if necessary, it will shut down to meetthis vital physiological need. Anecdotal, observa-tional, and subjectivelogbook data indicate that long-haul operations can involve the occurrenceof spontaneous and uncontrolled sleep episodes. A NASA/FAA study was conductedto determine the effectiveness of a planned cockpit rest period to maintainand/or improve subsequent alertness and performance during long-haul flightoperations (ref. 30).
The planned cockpit rest study involved regularly scheduled, three-person,non-augmented, commercial B747-200 transpacific flights. The middle fourlegs of an eight-leg, twelve-day trip schedule were studied. The studylegs involved two day flights and two night flights, and two eastward andtwo westward flights. Each flight was about 9 hours in length followedby an average layover of 24 hours. Volunteer flight crew-members were randomlyassigned to one of two groups. The twelve Rest Group crew-members wereeach allowed a scheduled 40-minute rest opportunity, one at a time, duringthe low workload, cruise portion of flight. The rest periods were takenin their seats. The nine No-Rest Group crewmembers each had a 40-minutecontrol period identified, but were instructed to continue their usual flightactivities during this period.
Before, during, and after the twelve-day trip schedule, flight crewmemberscompleted the Pilot Daily Logbook. This provided self-reported informationabout duty periods, sleep periods, fatigue ratings, and so on. Each crewmemberalso wore an actigraph, a small wristwatch-size device, that provides objectiveinformation about an individual's 24-hour rest/activity cycle. During thefour study trip legs, flight crewmembers' brain and eye movement activitieswere monitored to physiologically determine sleep during the rest opportunityand to evaluate subsequent alertness. Crewmembers were also evaluated witha vigilance performance test and reported their levels of alertness andmood. Crewmembers in both groups were evaluated with exactly the same measuresand procedures.
The first question was, "When given the opportunity, would flightcrewmembers sleep during the 40-minute rest period?" On 93% of thesleep opportunities, Rest Group crewmembers slept. On average, they fellasleep in 5.6 minutes and slept for 25.8 minutes. The next question waswhether this nap was associated with a subsequent maintenance or improvementin alertness and performance compared to the No-Rest Group. The Rest Groupmaintained consistent vigilance performance on night flights, at the endof a flight leg, and after four consecutive flight legs; the No-Rest Groupshowed decrements. Also, physiological alertness was examined by analyzingthe subtle brain and eye movement changes that indicate sleepiness. Thefinal 90 minutes of flight (about 60 minutes prior to top of descent, throughdescent and landing) was analyzed for the occurrence of physiological microevents,lasting 5 seconds or longer, which are indicative of decreased alertness. These physiological microevents are similar to "microsleeps"that many individuals have experienced when fighting sleepiness and attemptingto maintain wakefulness. The nine No-Rest Group crewmembers had twice asmany microevents, including twenty-two during descent and landing, thanthe twelve Rest Group crewmembers, who experienced no microevents duringdescent and landing.
Another provocative finding emerged from analysis of the 40-minute controlperiod for the No-Rest Group crewmembers. On five occasions, crewmembersfell asleep during the 40-minute period when they had been instructed tomaintain their regular flight activities. These sleep episodes lasted froma couple of minutes to 14-minutes. These physiologically documented sleepepisodes occurred in a NASA/FAA study of fatigue, when volunteers were beingphysiologically monitored and observed by two NASA researchers on the flightdeck. Clearly, this is a situation where crewmembers would have been motivatedto maintain their usual flight activities for the 40-minute period. Thissupports previous information that regardless of training, professionalism,or having the "right stuff," extreme sleepiness can precipitateuncontrolled and spontaneous sleep.
Therefore, the "NASA nap" was associated with improved alertnessand performance compared to a No-Rest Group. Based partly on these findings,the FAA requested an industry/government working group to draft an AdvisoryCircular that would provide guidelines for controlled rest on the flightdeck. This proposal is currently under review by the FAA. In the future,controlled rest on the flight deck may be another operational countermeasureavailable to maintain alertness and performance during long-haul flightoperations