Looking at Stress and Fatigue in Aviation
Unsolved Issues: Part IV, Amber Berlin
Think you wouldn’t drink and fly a plane? You might be doing something similar without even realizing it. This article reveals the exact hour limit when your long day becomes intoxicating, and why you wouldn’t realize when it happens. Understanding your limits, and how they’re affected by stress and fatigue in aviation is knowledge that will make you a better pilot, and may even save your life.
Scientists currently believe that stress and fatigue in aviation are developed from a variety of sources, and no one is immune from them. Although the effects on the body are different, excessive exposure to mental stimulation produces the same measurable results as extensive manual labor and leads to a decrease in the ability to carry out tasks. (FAA Publication, Medical Facts for Pilots, 2002). Any person operating in a fatigued condition, regardless of the cause of fatigue, will exhibit the same problems.
Historically, research indicates obtaining adequate sleep is the best way to prevent or resolve stress and fatigue in aviation. However, because of the complex nature of the body’s response to a variety of factors other than sleep deprivation, adequate sleep is not to be considered a complete solution in fatigue management. Because of the dynamic environments in which we operate, our body’s experience a myriad of situations and events which lead to the effects of fatigue. Sleep deprivation directly contributes to fatigue because the body does the majority of its recovery during sleep.
Sleep deprivation refers to no sleep or a reduction in the usual total sleep time. Various amounts of sleep deprivation have shown to reduce cognitive function and can negatively impact performance levels. Sleep deprivation affects the performance of sustained attention tasks and manifested itself in a higher number of omission errors (Fafrowicz et al., 2010). These omission errors, defined as “lapsing or failing to respond in a timely fashion to a presented stimulus”, are related to micro sleeps and increase under high fatigue conditions (p.940).
Durmer and Dinges (2005) conducted several partial sleep deprivation studies, which indicate a suboptimal sleep dose has measurable effects on concentration and the performance of cognitive tasks. Reports have shown that the average sleep obtained by a pilot is approximately 6 hours per night. Over a two-week span, the body’s response to sleeping only 6 hours per night is similar in cognitive ability to operating under an entire night of sleep loss. Think about this for a moment…let it sink in…the average pilot is operating on a day-to-day basis, flying their aircraft with the same cognitive ability as if they’ve just been awake all night.
For those individuals receiving only 4 hours of sleep during the same span, the cognitive effects are comparable to an entire weekend of sleep loss (Durmer and Dinges, 2005). Hopefully, you are all getting more than 4 hours a night. While no cognitive deficits occurred for 8 hours of sleep per night, 1 hour of sleep loss per night causes reduced waking alertness, and 2 hours of sleep loss can “significantly affect both alertness and performance” (Rosekind, Co, Gregory, and Miller, 2000, p,4). These studies have shown there is a consistent decline in cognitive ability due to sleep loss and denote the importance of attaining the recommended 8 hours of sleep per night.
It is also known that subjective reports of fatigue are typically underestimated, as individuals “are often sleepier than they report” (Overton and Frazer, 2013, p.219). Studies using physiological measures of sleepiness have shown that people can, “report a high level of alertness during the day and yet still exhibit significant physiological sleepiness” (Neri, Dinges, and Rosekind, 1997, p.11). This alludes to the role of environmental stimulation in the individual perception of stress and fatigue in aviation and identifies a cognitive disassociation between how the subject feels and their actual physiological state. This disassociation inhibits the pilot from realizing when they are fatigued, thereby making it impossible to accurately report their fatigue level. If you ask the pilot, they will feel rested enough to fly, even if they are not. Without as much environmental stimulation, such as in the early hours of the morning, their actual level of physiological sleepiness may make it impossible to stay awake.
Kuo et al. (1998) found that “during chronic partial sleep deprivation, subjective sleepiness increased during the first week, but decreased during the second week, suggesting that subjects believed they were adapting to the effects of sleep loss, whereas performance measures indicated that they were not: (Kloss, Szuba, and Dinges, 2012, p.1900). Because of this illusion of adaptation to the effects of chronic partial sleep deprivation, pilots may believe they are fit for duty when in fact they are experiencing a dangerous level of fatigue. While most pilots are not subject to periods of acute total sleep deprivation, chronic partial sleep deprivation is a highly common occurrence in the aviation operational environment. Because of the effects of fatigue on perception, when “attempting to judge how sleepy an individual is, the worst person to ask is that individual” (Neri, Dinges, and Rosekind, 1997, p.11).
Cumulative Sleep Loss
Another factor which must be considered is cumulative sleep loss, or sleep debt. Sleep debt is the accumulation of missed sleep over several days or weeks, which an individual has not had the opportunity to make up. Any sleep of less than 8 hours per night may result in a sleep debt. If you miss three hours of sleep on Wednesday, and one hour of sleep on Thursday, by Friday you are operating under 4 hours of missed sleep. According to one study, it takes more than the recommended 8 hours of sleep to make up a sleep debt, as sleeping 8 hours merely fulfills the daily requirement for sleep, thus “two nights of recovery sleep are typically needed to resume baseline levels of sleep structure and waking performance and alertness” (The Royal Aeronautical Society, n.d.).
Cumulative sleep loss can be any combination of total or partial sleep loss, and cognitive effects have been shown in as little as 1 hour of missed sleep per night. Studies have shown that “4 or more days of partial sleep restriction involving less than 7 hours sleep per night resulted in cumulative adverse effects on neurobehavioral functions” (Durmer and Dinges, 2005, p.123). Along with the increase in sleep debt, there is also an increase in attention lapses and daytime sleep propensity, and a decrease in cognitive speed and accuracy on working memory tasks (Van Dongen, Maislin, Mullington, & Dinges, 2003; Drake et al., 2001; Dinges et al., 1997; Belenky et al., 2003).
Hours of Continuous Wakefulness
Hours of Continuous Wakefulness refers to the number of hours since the last sleep episode. Durmer and Dinges (2005) suggested that there is a critical period of stable wake time within each circadian cycle, after which neurocognitive deficits occur. They have statistically estimated the optimal sleep time to be 8.16 hours, with the corresponding 15.84 hours of wakefulness completing the 24-hour circadian cycle (Durmer and Dinges, 2005).
According to current research, there is a drive for sleep which increases progressively with the duration of time the individual spends awake. In a study comparing the cognitive effects of this homeostatic sleep drive and the cognitive effects of alcohol, The Royal Aeronautical Society found, “after 17 hours of continuous wakefulness, cognitive psychomotor performance decreased to a level equivalent to a blood alcohol concentration of 0.05%”, and “after 24 hours of continuous wakefulness performance was approximately equal to a blood alcohol concentration of 0.10%” (n.d.). Additionally, NTSB investigations have found that flight crews on long duty days (a shift of more than 13 hours) exhibit a disproportionate amount of accidents when compared to those on short duty days (a shift of less than 13 hours) (Federal Aviation Administration, 2010).
The legal limit of alcohol intoxication to operate a vehicle in most States is 0.08%. To put it into perspective, if you have been awake for 17 hours, your brain responds as if you were over halfway drunk by vehicle standards, and over the limit to fly an aircraft. The federal blood alcohol limit for pilots is 0.04%. Should you be flying 17 hours after waking up? Absolutely not. This poses a problem for pilots working evening shifts, as they probably woke up early in the day, and were awake all day, and then started their shift, putting them in a situation where they are operating under the influence of fatigue. The FAA has taken a hard line against alcohol, adhering to the strict limit of 0.04%. Out of the thousands of pilots tested each year, only a few of them fail the breathalyzer, and alcohol-related crashes are rare. Since pilots cannot come to work drunk, it makes sense to limit their operational usefulness if they are known to have been awake for a duration in which intoxicating effects are present, regardless of the cause. Schedulers should also be aware of this limitation caused by stress and fatigue in aviation, and not assign pilots more flights than safety permits, based on the duration of time they’ve been awake.
Will a nap at noon be as good as a nap at 3pm? Find out as we continue our quest for cognitive excellence in Unsolved Issues: Part V – A Simple Way For Pilots To Address Aviation Fatigue.
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Featured Image: Kent Wien