U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Basics of Aviation Fatigue Date: 06/07/10 Initiated by: AFS-200 AC No: 120-100 Change:
1. PURPOSE. This advisory circular (AC):
• Summarizes the content of the FAA international symposium on fatigue, “Aviation Fatigue Management Symposium: Partnerships for Solutions”, June 17-19, 2008;
• Describes fundamental concepts of human cognitive fatigue and how it relates to safe performance of duties by employees in the aviation industry;
• Provides information on conditions that contribute to cognitive fatigue; and
• Provides information on how individuals and aviation service providers can reduce fatigue and/or mitigate the effects of fatigue.
2. APPLICABILITY. This AC is not mandatory and does not constitute a regulation.
3. DEFINITIONS.
a. Circadian Challenge. Circadian challenge refers to the difficulty of operating in opposition to an individual’s normal circadian rhythms or internal biological clock. This occurs when the internal biological clock and the sleep/wake cycle do not match the local time. For example, the sleep period is occurring at an adverse circadian phase when the body wants to be awake. Engaging in activities that are opposite of this natural biological system represents the circadian challenge (e.g., night work, shift work, jet lag).
b. Cognitive Performance. Cognitive performance refers to the ability to process thought and engage in conscious intellectual activity, e.g., reaction times, problem solving, vigilant attention, memory, cognitive throughput. Various studies have demonstrated the negative effects of sleep loss on cognitive performance.
c. Circadian Rhythm. A circadian rhythm is a daily alteration in a person’s behavior and physiology controlled by an internal biological clock located in the brain. Examples of circadian rhythms include body temperature, melatonin levels, cognitive performance, alertness levels, and sleep patterns.
d. Circadian Synchrony. Circadian synchrony occurs when a person’s internal biological clock matches the local external time cues (e.g., light/dark cycle, social interaction). In other words, sleep opportunities occur when the body wants to sleep and waking activities take place when the body is promoting wakefulness and alertness.
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e. Endogenous Circadian System. The endogenous circadian system refers to the biological clock in the brain that programs humans to be awake during the day and asleep at night. It also regulates alertness, performance, and sleepiness levels through the 24-hour day.
This internal system persists independent of periodic changes in the external environment based on the time of day and can be modified or reset by environmental inputs such as light.
f. Fatigue. Fatigue refers to a physiological state in which there is a decreased capacity to perform cognitive tasks and an increased variability in performance as a function of time on task.
Fatigue is also associated with tiredness, weakness, lack of energy, lethargy, depression, lack of motivation, and sleepiness.
g. Fatigue Risk Management System (FRMS). FRMS is a scientifically based, data-driven process and systematic method used to continuously monitor and manage fatigue risks associated with fatigue-related error. FRMS can be, but is not necessarily required, a fundamental part of an organization’s Safety Management System (SMS).
h. Homeostatic Sleep Drive. The homeostatic sleep drive is a fundamental neurobiological process involved in the timing and placement of sleep through a 24-hour day. Sleep is a vital physiological need and is critical to human existence. Good sleep is as important to health and well-being as proper nutrition and good exercise. The average adult sleep need is about 8 hours each day.
i. Safety Management System (SMS). An SMS is a coordinated, comprehensive set of processes designed to manage resources for optimal safety achievement. An SMS is a systematic approach to managing safety, including the necessary organizational structures, accountabilities, policies, and procedures representing a management approach to controlling risk.
j. Sleep Inertia. Sleep inertia (also termed sleep drunkenness) refers to a period of impaired performance and reduced vigilance following awakening from the regular sleep episode or nap. This impairment may be severe, last from minutes to hours, and be accompanied by micro-sleep episodes.
k. Time Givers. This concept comes from the German word, “zeitgeber,” which represents any external cue that entrains or aligns humans’ internal time-keeping system with external stimuli. The strongest zeitgeber is light. Other time givers include temperature, social interactions, pharmacological manipulation, and eating/drinking patterns.
l. Window of Circadian Low (WOCL). Individuals living on a regular 24-hour routine with sleep at night have two periods of maximum sleepiness, also known as “WOCLs.” One occurs at night, roughly from 3 a.m. to 5 a.m., a time when physiological sleepiness is greatest and performance capabilities are lowest. The other is in the afternoon, roughly from 3 p.m. to 5 p.m.
4. RELATED READING MATERIAL.
• Proceedings of the Aviation Fatigue Management Symposium: Partnerships for Solutions, June 17-19, 2008.
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• Caldwell, J. A.; Caldwell, J. L. Fatigue in Aviation: A Guide to Staying Awake at the Stick (Studies in Aviation Psychology and Human Factors). Ashgate Publishing Limited;
2003.
• Dinges, D. F.; Graeber, R. C.; Rosekind, M. R.; Samel, A.; Wegmann, H. M. Principles and guidelines for duty and rest scheduling in commercial aviation. Moffett Field, CA:
NASA Ames Research Center; 1996. Report No.: 110404.
• Dinges, D.; Mallis, M.; Bankls, S. Aircrew Fatigue & Circadian Rhythmicity (Chapter
13). In Elsevier, E. Salas, T. Allard, & D. Maurino, (Eds), Human Factors in Aviation (2nd edition), Academic Press; 2009.
• Rosekind, M. R.; Gander, P. H.; Connell, L. J.; Co, E. L. Crew Factors in Flight Operations X: Alertness Management in Flight Operations Education Module. (NASA Technical Memorandum 2001-211385 DOT/FAA/AR-01-01). Moffett Field, CA: NASA Ames Research Center.
5. BACKGROUND. The traditional definition of fatigue is a physiological state in which there is a decreased capacity to perform cognitive tasks and an increased variability in performance.
While fatigue is often attributed to periods of extended wakefulness in which ample recovery sleep is not obtained, research has shown that performance and alertness levels are largely influenced by the complex interaction between sleep and the 24-hour biological clock (circadian rhythm). In addition, time on task can further increase fatigue. Although research has established empirically-based knowledge of sleep and circadian principles, current regulations, policies, and practices do not incorporate this scientific research 31. Thus, the challenge of fatigue among aviation employees has steadily increased along with fatigue-related concerns over air safety, due to increasingly complex operations that continue around-the-clock. Accident statistics, reports from pilots themselves, and operational flight studies all show that fatigue is a clear concern within aviation operations.
a. Risk Factors. Fatigue associated with aviation operations is a risk factor for occupational safety, performance effectiveness, and personal wellbeing. The multiple flight legs, long duty hours, limited time off, early report times, less-than-optimal sleeping conditions, rotating and non-standard work shifts, and jet lag pose significant challenges for the basic biological capabilities of pilots, crewmembers and shift workers. Humans simply are not designed to operate to operate effectively under the pressured 24/7 schedules that often define aviation operations, whether the operations are short-haul commercial flights, long-range transoceanic operations, or around-the-clock and shift work operations.
b. Personnel Reports. Short-haul (domestic) pilots commonly identify sleep deprivation and high workload as the main factors contributing to their fatigue. Long haul crewmembers generally attribute sleep deprivation and circadian disruption caused by multiple time-zone crossings as the main causes of fatigue 11. However, fatigue resulting from multiple flight legs, early wake times, consecutive duty days, insufficient recovery sleep periods, time demands, and jet lag are reported by both short-haul and long-haul flightcrew. Corporate/executive crews experience similar fatigue-related problems when compared with their commercial counterparts;
however, scheduling issues (multi-segment flights, night flights, late arrivals, and early
Page 4 Par 5 awakenings) are the most salient contributing factors to fatigue 75. Pilots also report weather and turbulence as contributing factors. Shift workers in aviation operations face a different set of fatigue drivers that affect the same underlying physiology. Because shift patterns can vary widely, the cause of shift work fatigue can be one or more of a variety of factors. These factors are night work that deprives a person of normal nighttime sleep, insufficient time between shifts to get recovery sleep, rotating shifts that force adjustment of the body clock to constantly changing sleep/wake patterns, and early start times that can shorten nighttime sleep.
6. THE FATIGUED BRAIN.
a. 24-Hour Biological Rhythms. Most organisms show daily behavioral and physiological changes that are cyclical across the day. A biological clock located in the brain controls these 24-hour cycles in humans—circadian rhythms 64 (see Figure 1). Circadian rhythms result in two periods of sleepiness (nadirs) throughout the day; the maximum sleep propensity occurs during the early morning (the latter half of the habitual sleep episode), and the second period of increased sleep propensity occurs in the mid-afternoon 26, 86. Time givers, environmental stimuli such as the light/dark cycle and social cues also influence circadian rhythms. These time givers help to synchronize (entrain) circadian rhythms to the 24-hour day. However, in the absence of external time cues, the circadian cycle is slightly longer with a free running period of 24 to 28 hours 21. Core body temperature is a reliable marker of fluctuations in the 24-hour clock.
People have increased feelings of sleepiness and reduced performance levels when core body temperature reaches its low point, or nadir 23. This rhythm affects many dimensions of cognitive performance (see below) but the most obvious change is an increase in reaction time and tendency to have lapses in attention often associated with short sleep attacks (micro-sleeps).
b. Sleep. In addition to the circadian component, the daily pattern of sleep strongly modulates alertness and cognitive performance.
(1) The brain requires a regular pattern of sleep to be fully functional and regulates the drive to sleep in order to restore alertness and performance.
(2) The average person usually needs about 8 hours of sleep per day to remain fully alert and functional.
7. FOUR SLEEP-RELATED PROCESSES TO UNDERSTAND.
a. Sleep Regulation. The drive for sleep increases over time since the last sleep period and with any cumulative deficit in sleep relative to the average 8-hour day requirement. As a consequence, the sleep drive is at its lowest point in the morning, upon awakening, and as the day progresses, the drive to sleep increases and the ability to sustain attention and engage in cognitive activities decreases. Once sleep begins, this drive gradually decreases until awakening.
The system is homeostatic in the sense that the more a person is deprived of good quality sleep (relative to the nominal 8-hour requirement), the stronger the drive for sleep. The two main fatigue processes—24 hour (circadian) rhythm and sleep regulation—combine to produce dynamic changes in sleep tendency and ability to maintain stable alert performance across a 24-hour period and across days (see Figure 1).
Par 7 Page 5 FIGURE 1. CIRCADIAN PATTERN OF PERFORMANCE Time Window of Circadian Low (WOCL) Secondary Window of Circadian Low (SWOCL) 8 hr sleep awake nap napsleep 9 hr recovery sleep 9 hr recovery sleep awake awake awake awake awake Morning Circadia n Peak Evening Circadia n Peak Night Duty Period Day 1 Day 2 Day 3 Day 4 Day 5 Performance Percent Effectiveness
b. Elevated Sleep Drive. For the average person, the daily upswing in alertness produced by the circadian system tends to offset the decrease in alertness produced by depletion of the sleep regulatory process. The result is roughly constant reaction time and lapses during the first 16 hours of the day 85. After about 16 hours of continuous wakefulness, most adults begin to notice reductions in the speed of performance and in alertness levels 87. However, a prior history of insufficient sleep quantity and quality can magnify the changes in behavior and alertness.
Consequently, three factors can result in elevated homeostatic sleep drive:
• Increasing time continuously awake,
• Inadequate sleep duration for one or more consecutive days,
• Physiologically disrupted (fragmented) sleep due to medical conditions
(e.g., untreated sleep disorder such as obstructive sleep apnea) or environmental factors (e.g., attempting to sleep upright or in an uncomfortable environment).
c. Desynchronization. The timing of sleep and wakefulness of most humans, under natural conditions, is consistent with the circadian control of the sleep cycle and all other circadian-controlled rhythms. However, people working in a developed society override their internal biological clock and attempt to sleep at times that are not always consistent with the biological drive to sleep. For example, when individuals travel rapidly across time zones or work the night shift, the sleep/wake cycle is out of phase with the biological rhythms controlled by the circadian clock. This can adversely affect both alertness while awake and at work, and the ability to achieve restorative sleep 23. This sort of disruption of circadian synchrony can result in
Page 6 Par 7 difficulties, such as impaired cognitive function, sleepiness, altered hormonal function, and gastrointestinal complaints 72, 76.
d. Sleep Inertia. This sleep-related process causes a temporary degradation in performance immediately after awakening. The degradation or loss of alertness is dependent on depth of sleep at the time of awakening. The degradation dissipates, after awakening, on a time scale ranging from minutes to a few hours (see Figure 1). Sleep inertia causes a feeling of drowsiness or lethargy and can be measured as a noticeable change in reaction time and potential for lapses in attention. The duration and severity of sleep inertia is related to the depth of sleep at the time of awakening. It tends to be greater after short sleep periods of an hour or two, when the need for sleep is not fully satisfied, or after sleep when the person is carrying a large sleep debt from prior sleep restrictions 10, 23, 88.
8. EFFECTS OF FATIGUE ON HUMAN PERFORMANCE.
a. Cognitive Performance is a Safety Critical Process. Maintaining optimal alertness and neurobehavioral functioning in operational environments is critical for achieving high levels of safety, efficiency, and success. High levels of alertness and performance are necessary to operate complex technology and machinery as well as to make critical task decisions on a sustained basis. Individuals working erratic schedules experience conflicts between the biological circadian rhythm and environmental time cues and work demands. This physiological conflict can cause a sense of drowsiness (subjective fatigue), mood changes, performance degradation, and physiological upset 44. Two adverse effects of the circadian conflict between the sleep/wake pattern and the biological rhythm worsen performance levels and sleepiness:
• Trying to sleep when a person’s biology is highly energized, and
• Attempting to maintain alertness and high cognitive functioning at a time when a person’s biological clock is programming the body to sleep.
(1) Figure 1 is a diagram of performance across days and illustrates the circadian patterns in performance (dark line). There are two peaks in performance—one in the late morning and one in the early evening. After about 7 p.m., the circadian rhythm in performance begins to decline and if the person stays awake throughout the night, as on Day 3, will experience a strong early morning low (nadir) in performance referred to as the WOCL—generally between 3 a.m. and 5 a.m. This is a period of generally low alertness and performance and elevated operational risk; it is also the optimal time to obtain restorative sleep. There is a secondary dip in alertness in the early afternoon, referred to as the secondary WOCL, which is a period of increase drowsiness and performance risk lasting for several hours. The secondary WOCL is a relatively good time to obtain a brief nap, if additional sleep is necessary. The exact synchronization of this rhythm with the clock varies somewhat from individual to individual, and the relative size of the morning and evening peaks in performance also varies between people 43,
52. We refer to those who feel best in the morning as “larks” and those who feel most energized in the evening as “owls.” The person diagrammed in Figure 1 is neutral with approximately symmetrical peaks in morning and evening performance.
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(2) In Figure 1, note that on Day 3, after missing a single night of sleep and having a 3 hour nap in the morning, evening performance is about 10 percent below what it would be on a normal day, see Day 1. The effect of this single period of sleep deprivation has lasting effects;
after 2 days of recovery with 9 hours of sleep, performance is not fully recovered. For this hypothetical person, it would take an additional day to recover full capability.
b. Objective Performance Changes. Laboratory and simulator studies have documented changes in all aspects of neurobehavioral functioning associated with fatigue and sleep loss. As little as 2 hours of sleep loss a day can result in impaired performance and alertness 19, and the decrements are especially apparent during late night and early morning hours 62. Although the degree to which sleep loss affects individuals and how that sleep loss is expressed varies greatly among people 36, 48, 87, all fatigued and sleepy individuals begin to have decrements in attention and vigilance. Total sleep deprivation is not necessary to produce profound changes in performance. When sleep is restricted over successive days, serious cumulative performance deficits can occur in less than 1 week 3, 34, 87. For example, 4 days of sleep restriction from 8 hours to 5 hours sleep per night can lead to performance changes as severe as that produced by 40 hours of total sleep deprivation. Lapses, brief periods in which an individual fails to respond to a stimulus, increase in frequency and duration as sleepiness levels increase 29, 32. Other notable and observable performance decrements are 6, 18, 29, 32, 47, 65, 92:
• Slowed reaction times;
• Cognitive slowing (logical reasoning, mental arithmetic, coding-decoding);
• Difficulties maintaining situational awareness; and
• Impaired short-term memory.
c. Error Rates. People working regular 24-hour sleep/wake schedules with sufficient time to sleep will experience little change in performance and subjective fatigue during waking hours from 1 to 2 hours after awakening to 1 to 2 hours before sleep onset 22, 51. However, in many work environments, an increase in error rates and accident likelihood often occurs in the early afternoon, between 2 p.m. and 4 p.m. It is followed by a much larger increase in risk in the early morning hours between 2 a.m. and 4 a.m. that roughly coincides with the minimum of the circadian rhythm of core body temperature, as illustrated in Figure 1 10, 38, 54, 90.
d. Subjective Fatigue. People are not the best evaluators of their own alertness state.
People can lose awareness of their own fatigue levels. They are often sleepier than they report.
Although individuals report feeling increasing levels of sleepiness and fatigue with the progression of sleep loss, research has shown that these subjective estimates are unreliable 27, 49, 74, 95. Environmental conditions can affect subjective estimates. If an individual is in a highly engaged environment involving physical activity or interaction with other individuals, the person’s underlying sleepiness may not be noticeable and that person may rate him or herself as being more alert than his or her physiological responses indicate. This creates challenges for detecting and managing alertness and cognitive capability in operational environments because individuals often do not notice the gradual changes in performance until it is too late to take corrective action 33, 58.
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e. Mood. Fatigue can affect overall mood. Sleepy individuals often show deteriorations in mood and have reductions in amount and quality of communication and social interaction with other individuals. This can have serious consequences for crews and workgroups that rely on the exchange of information to provide mutual support and avoid errors. Workers often report workload as an important component of subjective fatigue. A flightcrew that performs many flight segments in a duty period will report more subjective fatigue than a crew that has a single flight segment taking the same amount of time 11. It is not known if this kind of fatigue results in reduced neurobehavioral functioning but it can combine with actual sleep loss to amplify the sense of fatigue reported by crewmembers. Time on task can also increase fatigue and accident risk. Studies show that after 8 hours of uninterrupted work, chances of error and accidents increase. Providing short breaks from work can greatly attenuate this effect 39.
9. SOURCES OF FATIGUE IN AVIATION.
a. Flight Operations. Sleep loss is one of the primary contributors to fatigue in flightcrew and is directly related to a variety of scheduling factors. Scheduling factors exist independent of the type of operation (i.e., short-haul, long-haul, ultra-long haul). For example, pilots report early morning report times and extended duty days as contributing to increased fatigue levels 11, 69.
Powell and colleagues found that it is not uncommon for pilot shifts to begin before 7 a.m. 69.
Thus, pilots are required to awaken near their low point in body temperature when sleep pressure is high and alertness levels are low. Additionally, sleep duration is often shortened due to sleep difficulties associated with trying to initiate sleep earlier the night prior in an attempt to compensate for an earlier wake time, which can be in conflict with the circadian system. In a sample of pilots observed over a 15-day period, sleep log and actigraphy data revealed that sleep prior to a duty day was reduced by almost 2 hours 70. Consequently, these pilots begin their flight with a sleep debt, and if sleep is shortened across consecutive days, the debt can be cumulative.
The sleep debt can also be aggravated by extended duty days involving work hours that continue into biological night. Thus, aviation schedules that require combinations of early start times and late evening (or early morning) end times make it difficult to maintain a regular sleep/wake cycle and reduce the opportunities available for recovery sleep. Research has shown that long-haul pilots commonly are awake longer than 20 hours, particularly on upwind flight segments 77, 41.
Schedules that involve short turn-around times between flights, resulting in an increase in takeoffs and landings and additional time constraints contributing to an increased workload, challenge short-haul crews. Short-haul pilots have reported that schedules consisting of 4 to 5 legs of flight are one of their more fatiguing schedules to fly 11.
b. Common Scheduling Factors. There are several common scheduling factors that can have major impact on sleep and alertness.
(1) Time Awake Prior to Duty. It is not just the duration of the duty day that is important. Time since awakening before the crew starts their duty period is equally, if not more, important. With the manufacturing of ultra long-range (ULR) aircraft and duty times approaching 20 hours, extended waking hours become unavoidable. Thus, time since awakening continues to become a more significant factor contributing to fatigue levels of flightcrew. Data collected from glass cockpit rated pilots during a simulated ULR flight found that pilots who departed at night, after being awake for at least 13.5 hours, had significantly reduced reaction times compared to pilots who departed during the morning hours, after about 3.5 hours of
Par 9 Page 9 wakefulness. Pilots in the nighttime group were particularly impaired during the first half of the flight due to both sleep and circadian factors that were promoting sleep. However, towards the end of the flight, as continuous hours of wakefulness increased, performance decrements were seen in both morning and nighttime departure groups 15.
(2) Layover Sleep Opportunities. Upon arrival at layover destinations, sleep opportunities often conflict with a pilot’s biological clock. For example, it can be local day at the layover city but the pilot’s biological clock is programming them for sleep because it is nighttime at their home base. Thus, a crewmember’s ability to obtain restorative recovery sleep is reduced before continuing duty. Obtaining optimal recovery sleep can also be a challenge if the layover city is in the same time zone as a pilot’s home base. While there may be an opportunity for a crewmember to obtain a nap during the afternoon secondary WOCL 17, 28 in preparation for a nighttime departure, there is no guarantee that the individuals will be able to obtain sleep. Some individuals may choose not to take advantage of the napping opportunity, and others may not be able to nap at odd hours of the day. Therefore, it is essential that scheduled layovers include multiple sleep opportunities, and thus sufficient recovery sleep, prior to the return flight. Total sleep time per 24-hour period is one of the most influential determining factor of performance, even when sleep time is an aggregate of major sleep period and a nap
(i.e., a split-sleep schedule) 63.
c. Opportunities for Recovery Sleep.
(1) Small reductions in sleep during a single trip may not cause serious changes in alertness and performance; however, if these small reductions are not made up after the trip prior to another trip, then the potential for serious accumulated sleep debt can occur. Studies have shown that an accumulation of only 1 hour of sleep loss per day over a week can have measurable effects on reaction time that may take several days to dissipate after the person returns to a normal sleep schedule 3, 87.
(2) Recent studies have also shown that rapid recovery from accumulated sleep debt requires that the person take deliberate steps to sleep more than the usual nominal 8 hours per day. It may take the average person several days of 9 hours of sleep or more to recover from a serious sleep debt. Hence, train staff to use recovery periods to sleep more than their usual amount to prevent accumulated sleep debt across an extended work or duty schedule.
d. Night Operations. Operational demands and advances in technology have led to the scheduling of flight operations throughout the 24-hour day. Thus, flight schedules require pilots to fly and shift workers to work on the “backside” of the clock, performing during times when they would normally be asleep. A survey of 739 airline pilots rated schedules involving nighttime inbound and outbound flights with daytime layover periods as especially fatiguing 11.
Nighttime flights involve increased continuous hours of wakefulness before duty, especially if the pilot was not able to obtain an afternoon nap. Thus, a person may frequently awaken in the morning and remain awake the entire day before duty begins in the evening for a night flight.
e. Circadian Shifts and “Jet Lag.”
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(1) Jet lag, resulting from crossing multiple time zones is another challenge in aviation operations, even for experienced flightcrews 53. The rate of adaptation after crossing multiple time zones depends on both the number of time zones crossed and the direction of travel.
Adjustment to westward travel is quicker than adjustment to eastward travel. Eastbound travel requires the individual to reduce their day to less than 24 hours (i.e., the circadian period is shortened), whereas westbound travel lengthens the circadian period 12, 76. Lengthening the day and staying awake longer than 24-hours is easier because of the inherent period of the circadian rhythm being slightly longer than 24 hours.
(2) Circadian adaptation is less likely to occur during long-haul trips of less than 3 days because it does not allow enough time for resynchronization of internal biological rhythms with the external environment; therefore, pilots can minimize circadian disruption by keeping the most similar sleep/wake schedule to their home time zone as possible 57. However, the exact timing of the circadian clock and rate of adaptation with multiple time zone crossings is not easily predictable, and thus a prescriptive formula for calculating a precise number of days needed for circadian adaptation in long haul crews is difficult without considering physiological, environmental, and operational factors 41.
f. Shift Work Operations. Fatigue is not only a challenge for flightcrew but is inherent in aviation shift work operations including Air Traffic Control and ground maintenance operations.
Due to around-the-clock aviation operations, air traffic and ground personnel are faced with shifted schedules that interfere with ‘normal’ sleep/wake cycles that permit night-time sleep and daytime work. Shift work has traditionally included only night work and rotating shift schedules, however, the modern definition is more comprehensive. It includes any schedule that can potentially affect both sleep and circadian rhythms. Specifically, any schedule outside the traditional 7 a.m.-6 p.m. timeframe can be categorized as shift work and such shifts are becoming increasingly common 84.
(1) Sleep difficulties are commonly associated with shift work because sleep disturbances and sleepiness are the most commonly reported complaints of shift workers 55, 66, 84. Although false, it is a not uncommon belief that as the shift worker adapts to their schedule over time and gets used to the non-standard shift, all of their problems associated with shift work are alleviated.
However, shift work is not just about sleep; it is a more complex issue.
(2) Shift work is not simply a term used to describe non-standard schedules. It also is associated with the disruption of an individual’s underlying physiology. Shift work requires people to override the internal biological clock that programs humans for daytime activity and nighttime sleep 9. This produces circadian misalignment, a condition in which the biological clock remains synchronized to the local time, driven by exposure to the local pattern of sunlight, but the sleep/wake cycle is out of sync with the local time. In other words, the sleep period is occurring at an adverse circadian phase, when the body is programmed to be awake. As a result, one can experience sleep difficulties (e.g., longer than normal times to fall asleep and early termination of sleep) resulting in continuous partial sleep deprivation and chronic sleep loss. The shift worker is further challenged by the fact that his or her sleep/wake cycle is constantly altered between work days and non-work days due to conflicting time cues from the day/night cycle and a day-oriented society (e.g., keeping the same day schedule as the family on non-work days).
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(3) A number of common scheduling factors disrupt sleep and circadian rhythms and affect alertness and performance of shift workers in aviation environment. These work schedule factors might include early start times, variable work periods, insufficient recovery time, consecutive work periods, and on-call status, among others.
10. PREDICTIVE MODELS OF SLEEP, FATIGUE, AND PERFORMANCE.
a. Biomathematical Models of Fatigue and Performance. Predictive modeling algorithms can forecast the effects of fatigue on performance and risk. These models can be an important tool for work scheduling. Modeling can help minimize fatigue related errors, incidents and accidents during flight operations or shift work environments. These models are scheduling tools that can help to quantify the impact of underlying interaction of sleep and circadian physiology on performance levels.
(1) The theoretical framework of most of these models is the two-process model of sleep and alertness 1 that incorporates mathematical simulations of the two major processes in Figure 1, sleep regulation and circadian variation. Several initiatives are currently underway that incorporate the application of biomathematical modeling software to:
(a) Predict the times that neurobehavioral functions and performance will remain constant,
(b) Establish ideal time periods for maximal recovery sleep, and
(c) Determine the cumulative effects of different work-rest schedules on performance 59.
(2) Such models can serve as useful tools when evaluating the placement and timing of critical flight phases to assist in the scheduling of in-flight rest periods and layover sleep opportunities. While most of the biomathematical modeling software show promise in the prediction of performance, it is important that models used for scheduling have demonstrated validity for prediction of operational risk when applied with real world data 30. In other words, one can plan with the model but must confirm the effect.
(3) The Federal Railroad Administration has sponsored a study that validated the ability of a biomathematical model, considering only work schedule information, to predict the increased risk of accidents with reduced cognitive effectiveness and increased fatigue.
Subsequent work demonstrated that the same model could predict an increased severity of accidents (property damage) with increased fatigue 50.
b. Limits of Modeling. Currently, predictive fatigue models describe the effects of sleep history and circadian rhythms on the performance of an average person, assuming that the person requires about 8 hours of sleep per night to remain fully rested and has a regular circadian rhythm that favors neither the morning nor the evening for peak cognitive alertness. Efforts are underway to design procedures to tailor fatigue models to the characteristics of individuals but those tools require performance information from the individual to direct the changes to the model. The availability of data and assumptions about initial conditions also limits modeling
Page 12 Par 10 application. Any model must start at some point in time and must assume some initial level of fatigue prior to the period of examination. Most models assume a fully rested person at the start.
c. Sleep Estimation. While measurement of sleep is possible for research studies, it is seldom available for operational assessment or forecasting of fatigue. All two-process physiological models for fatigue require an algorithm to estimate the amount of sleep obtainable under a particular work schedule, since it is sleep that restores performance and wakefulness that exhausts performance. Sleep estimation requires a thorough understanding of how workers adapt sleep patterns to the demands of their work schedule. Sleep estimation assumptions that work well for one group of workers may not work well to predict sleep in another group of workers;
hence, sleep estimation algorithms that drive fatigue models must be validated for the specific work group for reasonable concordance with their typical sleep patterns. This is particularly important for work groups adapted to night work, split shifts, or on-duty sleep schedules.
11. STRATEGIES TO MITIGATE THE EFFECTS OF FATIGUE.
a. Recovery Sleep. We know that the longer one works without adequate opportunity to sleep, the greater the need for recovery sleep to prevent an accumulation of fatigue across duty periods. To some extent, FAA regulations codify this principle and specify off-duty rest periods that are proportional to the prior flight time. We now recognize that what is most important from a fatigue perspective is the total duration of duty that limits sleep opportunities. In general, the longer the period of duty, the more likely that duty will interfere with the nominal requirement to obtain 8 hours of sleep per day. However, circadian factors are very important in this calculation;
10 hours of duty starting at 8 a.m. will interfere less with normal nighttime sleep than 10 hours of duty starting at 10 p.m. Hence, the calculation of needed recovery sleep is complex since it depends on the circadian timing of the duty periods. Furthermore, the effects of sleep restriction are cumulative and the longer the period of sleep restriction, the greater the need for recovery sleep. There is some evidence that prolonged sleep restriction can lead to a slowing of the recovery process.
(1) The duration of the recovery sleep will depend on the amount of sleep loss that occurs as a result of a sequence of duty periods. The amount of recovery sleep required to fully restore performance will increase with the total amount of missed sleep since the person was fully rested, that is, had sufficient sleep to be fully alert.
(2) The amount of recovery sleep required to repay the deficit from sleep restriction is related to the total amount of sleep debt. Since the sleep regulatory process is adaptive, the amount of sleep required to make up a deficit is less than the total number of hours of sleep missed; it does not take 8 additional hours of sleep to make up for an 8-hour accumulated sleep debt. However, since it takes 8 hours of sleep to balance a normal day of wakefulness, it will require more than 8 hours of sleep per recovery day to repay the debt. In general, if a person has experienced several days of sleep restriction below the nominal requirement of 8 hours per day, full recovery of performance may require several days of 9 hours or more sleep per day.
(3) For recovery sleep to be an effective mitigation, the schedule must permit an adequate number of recovery nights of sleep and the employee must be trained to use those recovery days