Fatigue has been an issue in long-haul flight operations since the first transoceanic crossing by Charles Lindbergh. Today, modern aircraft have proven the capability to fly farther. The aircraft are highly automated, and require fewer crew members for operation.
While aircraft and the operational demands of the aviation industry have evolved, human requirements for sleep have not. Therefore, the physiological capabilities and limitations of the human operator are the main focus to maintaining the safety margin in long-haul flight operations. I will address some of the physiological limitations that underlie fatigue, highlight findings regarding fatigue in long-haul operations. Physiological limitations that Underlie Fatigue have been heavily studied Since the mid-1950s, there has been extensive scientific research on sleep, sleepiness, circadian rhythms, sleep disorders, dreams, and the effects of these factors on waking alertness and human performance. Some of the basic scientific findings regarding human sleep, sleepiness, and circadian rhythms that have emerged over the past fifty years and are critical to understanding the physiological limitations that underlie fatigue in flight operations.
Some of the significant information is presented as a foundation for understanding the role of fatigue in long-haul operations. 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, active physiological state that is essential to human survival. Like human requirements for food and water, sleep is a vital physiological need. When individuals are deprived of food and water, the brain provides specific signals-hunger and thirst drives 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 a person to obtain sleep; it is a signal that a specific requirement has not been met. Eventually, when deprived of sleep, the human brain can spontaneously, in an uncontrolled fashion, shift from wakefulness to sleep, in order to meet its physiological need for sleep. The sleepier the person, the more rapid and frequent these actions of sleep into wakefulness are. These spontaneous sleep episodes can be very short (micro sleeps lasting only seconds) or extended (lasting minutes).
At the onset of sleep, an individual disengages from the external environment, becoming unresponsive to outside information. Therefore, even micro-sleep can be associated with a significant performance lapse when an individual does not receive or respond to external information. With sleep loss, these uncontrolled sleep episodes can occur while standing, operating machinery, and even in situations that would put an individual at risk, such as driving a car.
How much sleep does an individual need? An individual requires the amount of sleep necessary to achieve full alertness and the highest level of functioning during waking hours. There is a range of individual sleep needs and, though most adults will require about 8 hours of sleep, some people need 6 hours while others require 10 hours to feel wide awake and function at their peak level during wakefulness. Sleepiness Affects Waking Performance, Vigilance, and Mood. Sleepless creates sleepiness and often this sleepiness is dismissed as a minimal nuisance or as easily overcome. However, sleepiness can potentially disrupt most aspects of human capability. Controlled laboratory experiments have demonstrated decrements in most components of human performance, vigilance, and mood as a result of sleep loss. Sleepiness can be associated with decrements in decision-making, vigilance, reaction time, memory, psychomotor coordination, and information processing. Research has demonstrated that with increased sleepiness, individuals demonstrate degraded performance despite increased effort, and report many differences regarding the outcome of their performance. Individuals report fewer positive emotions, more negative emotions, and an overall worsened mood with sleep loss and sleepiness.
Generally, sleepiness can degrade most aspects of human waking performance, vigilance, and mood. In the most severe instances, an individual may experience an 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 incorrectly process the importance of the information, may find decision making difficult, may make poor decisions, or may have to check and recheck information or activities due to memory difficulties. This performance degradation can be a direct result of sleep loss and the associated sleepiness and can play a major role in the occurrence of an operational incident or accident.
Sleep Loss Accumulates into a Sleep Debt. An individual who requires 8 hours of sleep and obtains only 6 hours is essentially sleep deprived by 2 hours. If the individual sleeps only 6 hours over four nights, then the 2 hours of sleep loss per night would accumulate into an 8-hour sleep debt. Estimates suggest that in the United States today, most adults obtain 1 to 1.5 hours less sleep per night than they actually need. During a regular work week this would translate into the accumulation of a 5- to 7.5-hour sleep debt going into the weekend; hence, the common phenomenon of sleeping late on weekends to compensate for the sleep debt accumulated during the week. Generally, recuperation from a sleep debt involves obtaining deeper sleep over two to three nights. Obtaining deeper sleep appears to be a physiological priority over a significant increase in the total hours of sleep. In other words, rather than sleeping 7.5 hours longer than normal on the weekend to “make-up” for the sleep debt accumulated during the week, the sleep-deprived person may sleep only slightly longer than normal; in a deeper sleep.
Sleepiness can be differentiated into two distinct components: physiological and subjective. Physiological sleepiness is the result of sleep loss: lose sleep, get sleepy. An accumulated sleep debt will be accompanied by physiological sleepiness that will drive an individual to sleep in order to meet the individual’s physiological need. Subjective sleepiness is an individual’s introspective self-report regarding the individual’s level of sleepiness. An individual’s subjective report of sleepiness can be affected by many factors, for example, caffeine, physical activity, and a particularly stimulating environment. However, an individual will typically report being more alert because of these factors. These factors can mask or conceal an individual’s level of physiological sleepiness. Therefore, the tendency will be for individuals to subjectively rate themselves as more alert than they may be physiologically.
This discrepancy between subjective sleepiness and physiological sleepiness can be largely significant. An individual might report a low level of sleepiness but be carrying an accumulated sleep debt with a high level of physiological sleepiness. This individual, in an environment stripped of factors that conceal the underlying sleepiness, would be susceptible to the spontaneous, uncontrolled sleep and the performance decrements associated with sleep loss. Humans, like other living organisms, have a circadian clock in the brain that regulates physiological and behavioral functions on a 24-hour basis. In a 24-hour period 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 periods of wakefulness and sleep, high and low body temperature, high and low digestive activity, increased and decreased performance capability, and so on. An individual’s circadian clock might be programmed to sleep at midnight, awaken at 8 AM, maintain wakefulness during the day, and then repeat the 24-hour pattern. The circadian rhythm of body temperature is programmed for the lowest temperature between 3 and 5 AM on a daily basis. When the circadian clock is moved to a new work/rest 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 can begin to adjust and may take from several days up to several weeks to adapt to the new environmental time. Also, the body’s internal physiological rhythms do not all adjust at the same rate, and therefore may be out of synch with each other for an extended period of time. Again, it can take from days to weeks; for all of the internal rhythms to come together in the new schedule or time zone.
There are some specific factors that can affect the circadian clock’s adaptation. Day/night reversal can confuse the clock so that the cues that help it adjust and maintain its usual physiological pattern is disrupted. Moving from a day to a night schedule and back to days can keep the clock in a continuous state of readjustment, depending on the time between schedule changes. For example, severe effects would accompany a 12-hour day to night to day schedule alteration. Another factor is crossing multiple time zones. Even though there is some flexibility for adjustment. Putting the circadian clock in a time zone, three or more hours off home time will require a reasonable amount of adaptation. Another factor can be the direction the clock is moved. Shortening the period is generally more difficult than lengthening the period, which is the natural rhythm of the circadian clock. Therefore, it can be more difficult to cross time zones in an eastward direction compared to westward movement. It can also be more difficult to move a work/rest schedule backwards over the 24-hour day compared to moving it forward. The associated difficulties of moving the clock, such as poor sleep, sleepiness, effects on performance, and so on, will be affected until the circadian clock adapts to the new schedule or time zone.
Scientific studies have revealed that there are two periods of maximal sleepiness during a usual 24-hour day. One occurs at night roughly between 3 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 through the 3-5 AM window of sleepiness. The afternoon sleepiness period can be masked by factors described previously, or present a window when individuals are particularly vulnerable to the effects of sleepiness. This also means that 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 circadian clock is programmed for wakefulness. However, individuals searching for specific windows when they are physiologically prepared to sleep, either for an extended sleep period or a strategic nap, can use these periods to their advantage. (sleepnet.com, 1995 2006)
At any given time, an individual’s need to sleep or, the ability to maintain alertness and vigilance, will be the result of an interaction between sleep and circadian processes. An individual’s ability to fall asleep quickly and obtain a good quantity and quality of sleep can be related to the prior amount of sleep and circadian time of day. An individual with no sleep debt attempting to sleep at a time of circadian wakefulness and alertness will have difficulty falling and staying asleep. However, an individual with a sleep debt attempting to sleep at a time of maximal circadian sleepiness will fall asleep quickly and easily maintain sleep. Also, an individual with a substantial sleep debt may be physiologically sleepy enough to override circadian factors and be able to fall asleep at a circadian time for wakefulness.
These two factors also interact to determine an individual’s level of physiological alertness and performance during waking hours. A third factor can also be a consideration: the number of hours of continuous wakefulness. An individual with a sleep debt, awake continuously for 20 hours, and working
through the 3-5 AM circadian period of maximal sleepiness will have difficulty maintaining alertness and performance. However, an individual who has obtained the required amount of sleep, has been awake for 10 hours, and is working through the 3-5 AM circadian low will probably have less difficulty maintaining wakefulness. Any one of these three factors can increase an individual’s vulnerability for a performance decrement. Two or three of the factors coinciding will increase the probability of a fatigue-related performance problem. It is critical to note that there are tremendous individual differences in these factors. There is a range of sleep needs, differences in physiological flexibility for adaptation of the circadian clock, and ability to tolerate sleep loss or circadian disruption. Therefore, while these fundamental properties of sleep and circadian processes are factors for all human physiology, there is a range of individual responses for any particular set of circumstances or operational demands. (sleepnet.com, 1995 2006)
There is no quick fix or magic bullet to address all of fatigue endured by long-haul flight operations. Unfortunately, there is no simple solution that will address all individuals, all operational demands, and all the technology currently involved in the aviation industry. Aviation requires24-hour operations and a challenge facing the industry is how to incorporate the scientific and physiological knowledge that currently exists into areas that will maintain the safety margin. Therefore, every arena where the knowledge can be applied should be examined for potential improvements.
Four general categories for examination include hours of service, scheduling, cockpit design and technology, and personal strategies. Hours of service are affectedly both by federal regulatory policies and contractual agreements. Scheduling is dictated by a complex variety of factors that are often distinctive to a particular airline’s operation. The automation evolution has brought tremendous advances to aviation, though its effects in a variety of domains remain unclear. There is also a variety of personal strategies that can be used to apply the current state of knowledge on a daily basis for flight crews. Each one of these areas should be examined for ways to incorporate scientific and physiological information about fatigue. The challenge is to minimize the adverse effects of any particular category and, wherever possible, use each one to maximize alertness and performance during flight operations.
We have proposed different alertness management strategies into two components: preventive
strategies and operational strategies. Preventive strategies are used prior to duty or on layover to minimize the adverse effects of the underlying physiological factors. These strategies include obtaining maximal quantity and quality of sleep prior to duty, scheduling sleep periods during layover, accounting for fatigue factors during trip scheduling, napping, maintaining good sleep habits, exercising, maintaining balanced nutrition, and others. Operational countermeasures are used in-flight to maintain alertness and performance during operations. Generally, these strategies may be more short-acting and serve to mask or conceal underlying sleepiness. These counter-measures include physical activity, strategic caffeine use, and social interactions. As previously described, the only mechanism to reverse a physiological sleep need is sleep. With sleep loss, the brain will signal its need to obtain sleep and if necessary, it will shut down to meet this vital physiological need. An observational and subjective logbook data indicate that long-haul operations can involve the occurrence of spontaneous and uncontrolled sleep episodes. A NASA/FAA study was conducted to determine the effectiveness of a planned cockpit rest period to maintain and/or improve subsequent alertness and performance during long-haul flight operations. (Rosekind, 1997)
The planned cockpit rest study involved regularly scheduled, three-person, on-augmented, commercial B747-200 transpacific flights. The middle four legs of an eight-leg, twelve-day trip schedule were studied. The study legs involved two day flights and two night flights, and two eastward and two westward flights. Each flight was about 9 hours in length followed by an average lay-over of 24 hours. Volunteer flight crew-members were randomly assigned to one of two groups. The twelve Rest Group crew-members were each allowed a scheduled 40-minute rest opportunity, one at a time, during the low workload, cruise portion of flight. The rest periods were taken in their seats. The nine No-Rest Group crewmembers each had a 40-minute control Period identified, but were instructed to continue their usual flight activities during this period. (Rosekind, 1997)
Before, during, and after the twelve-day trip schedule, flight crewmembers completed the Pilot Daily Logbook. This provided self-reported information about duty periods, sleep periods, fatigue ratings, and so on. Each crewmember also wore an actigraph, a small wristwatch-size device that provides objective information about an individual’s 24-hour rest/activity cycle. During the four study trip legs, flight crewmembers’ brain and eye movement activities were monitored to physiologically determine sleep during the rest opportunity and to evaluate subsequent alertness. Crewmembers were also evaluated with a vigilance performance test and reported their levels of alertness and mood. Crewmembers in both groups were evaluated with exactly the same measures and procedures.
The first question was, “When given the opportunity, would flight crewmembers sleep during the 40-minute rest period?” On 93% of the sleep opportunities, Rest Group crewmembers slept. On average, they fell asleep in 5.6 minutes and slept for 25.8 minutes. The next question was whether this nap was associated with a subsequent maintenance or improvement in alertness and performance compared to the No-Rest Group. The Rest Group maintained consistent vigilance performance on night flights, at the end of a flight leg, and after four consecutive flight legs; the No-Rest Group showed decrements. Also, physiological alertness was examined by analyzing the subtle brain and eye movement changes that indicate sleepiness. The final 90 minutes of flight (about 60 minutes prior to top of descent, through descent and landing) was analyzed for the occurrence of physiological micro-events, lasting 5 seconds or longer, which are indicative of decreased alertness. These physiological micro events are similar to “micro-sleeps” that many individuals have experienced when fighting sleepiness and attempting to maintain wakefulness. The nine No-Rest Group crewmembers had twice as many micro events, including twenty-two during descent and landing, than the twelve Rest Group crewmembers, who experienced no micro events during descent and landing.
Another provocative finding emerged from analysis of the 40-minute control period for the No-Rest Group crewmembers. On five occasions, crewmembers fell asleep during the 40-minute period when they had been instructed to maintain their regular flight activities. These sleep episodes lasted from a couple of minutes to 14-minutes. These physiologically documented sleep episodes occurred in a NASA/FAA study of fatigue, when volunteers were being physiologically monitored and observed by two NASA researchers on the flight deck. Clearly, this is a situation where crewmembers would have been motivated to maintain their usual flight activities for the 40-minute period. This supports previous information that regardless of training, professionalism, or having the “right stuff,” extreme sleepiness can precipitate uncontrolled and spontaneous sleep.