Fatigue on the flight deck: The consequences of sleep loss and the benefits of napping

Accident Analysis and Prevention 62 (2014) 309–318

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Fatigue on the flight deck: The consequences of sleep loss and the benefits of napping夽 Beth M. Hartzler ∗,1 Naval Medical Research Unit Dayton, 2624 Q Street, Building 851, Area B, Wright Patterson AFB, OH 45433-7955, United States

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Article history: Received 5 June 2013 Received in revised form 5 September 2013 Accepted 10 October 2013 Keywords: Strategic napping Aviation Safety Pilots Fatigue Sleep

a b s t r a c t The detrimental effects of fatigue in aviation are well established, as evidenced by both the number of fatigue-related mishaps and numerous studies which have found that most pilots experience a deterioration in cognitive performance as well as increased stress during the course of a flight. Further, due to the nature of the average pilot’s work schedule, with frequent changes in duty schedule, early morning starts, and extended duty periods, fatigue may be impossible to avoid. Thus, it is critical that fatigue countermeasures be available which can help to combat the often overwhelming effects of sleep loss or sleep disruption. While stimulants such as caffeine are typically effective at maintaining alertness and performance, such countermeasures do nothing to address the actual source of fatigue – insufficient sleep. Consequently, strategic naps are considered an efficacious means of maintaining performance while also reducing the individual’s sleep debt. These types of naps have been advocated for pilots in particular, as opportunities to sleep either in the designated rest facilities or on the flight deck may be beneficial in reducing both the performance and alertness impairments associated with fatigue, as well as the subjective feelings of sleepiness. Evidence suggests that strategic naps can reduce subjective feelings of fatigue and improve performance and alertness. Despite some contraindications to implementing strategic naps while on duty, such as sleep inertia experienced upon awakening, both researchers and pilots agree that the benefits associated with these naps far outweigh the potential risks. This article is a literature review detailing both the health and safety concerns of fatigue among commercial pilots as well as benefits and risks associated with strategic napping to alleviate this fatigue. Published by Elsevier Ltd.

1. Introduction Accidents occur because the behavior that causes them is not seen as risky Wagenaar and Groeneweg (1987, p. 596) The costs associated with fatigue due to sleep loss and poor sleep quality in aviation are tremendous, both in terms of money and the loss of life. Modern aircraft have strict maintenance schedules and are highly automated, and thus mishaps due to malfunctions or equipment failures are rare in comparison to mishaps attributed

夽 Disclosure: The views expressed in this article are those of the author and do not necessarily reflect the official policy or position of the Department of the Navy, Department of Defense, nor the U.S. Government. ∗ Tel.: +1 937 938 3921; fax: +1 9379048813. E-mail address: [email protected] 1 The author is an employee of the U.S. Government. This work was prepared as part of my official duties. Title 17 U.S.C. §105 provides that ‘Copyright protection under this title is not available for any work of the United States Government.’ Title 17 U.S.C. §101 defines a U.S. Government work as a work prepared by a military service member or employee of the U.S. Government as part of that person’s official duties. 0001-4575/$ – see front matter. Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.aap.2013.10.010

to impaired human performance (Avers and Johnson, 2011). One of the most frequently cited of these performance impairments is fatigue, which has been one of the foremost concerns of the National Transportation Safety Board (NTSB) for over 40 years (Federal Aviation Administration, 2012). Moreover, since 1990, the NTSB has made reducing fatigue one of the “most wanted” aviation safety improvements (Cabon et al., 2012; Federal Aviation Administration, 2012). However, even after years of recognizing the physical and cognitive impairments associated with fatigue due to sleep debt, as well as numerous improvements in available countermeasures, fatigue remains one of the primary physiologic factors implicated in aviation mishaps and general mistakes made by aircrews (Drury et al., 2012; Ritter, 1993). For example, fatigue was identified as a contributing or causal factor in both the 1988 and 1997 mishaps in Armenia and Guam, respectively, as well as the 1999 mishap in Little Rock and the 2004 mishap near the Kirksville Regional Airport (Aviation Safety Network; Goode, 2003; Hardaway and Gregory, 2005; Federal Aviation Administration, 2012). Each of these mishaps occurred during the approach and landing phases of flight, and those incidents in Guam and Little Rock happened early in the morning and late at night, respectively. Further, reviews of incident reports have revealed that approximately 20% of aviation

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incidents are related to fatigue (Petrie et al., 2004) and that from 1994 to 1998, there were an average of 45 mishaps every year attributed to fatigue due to poor scheduling (Goode, 2003). Examining the types of performance impairments which contributed to these mishaps, Lamond and Dawson (1999) found that as little as 24 h of continuous wakefulness was associated with a significant deterioration on measures of reasoning and vigilance. Further, the authors determined that this decline in performance was comparable to that observed among individuals with a blood alcohol content of .10. The dangers associated with this level of impairment are then compounded by the fact that fatigued individuals are typically unaware of how severely their performance has deteriorated (Banks and Dinges, 2007; Caldwell, 1997; Durmer and Dinges, 2005) and thus may believe that they are safe to fly when they are not. In these situations, when the drive to sleep is almost overwhelming, countermeasures such as caffeine can help to maintain performance and alertness during sleep loss. However, only an opportunity to sleep can actually reduce the accumulated sleep debt and give the body what it needs (Caldwell et al., 2009; Dinges et al., 1988; Hardaway and Gregory, 2005). Thus, the primary aim of this paper is to explain the use of shortened sleep periods, also known as strategic naps, which may be implemented during periods of otherwise continuous wakefulness. Specifically, this paper will detail both the recognized dangers associated with fatigue among aviators as well as the benefits of allowing pilots opportunities to nap to help compensate for sleep lost in relation to their duty periods. 2. Methods To examine the detrimental effects of fatigue among commercial pilots, the potential benefits of strategic napping, and the possible risks associated with napping, a series of online literature searches was conducted. These searches were conducted using Google Scholar, PsychINFO, PubMed, and Web of Science, with limitations set to articles and reports written in English and published between the years 1970 and 2013. The following keywords were used as search terms, both individually and in conjunction with each other: sleep; fatigue; nap(ping); rest; vigilance; attention; aviation; flight; performance; airline pilots; long-haul; in-flight; cognitive functions; accidents; risk. The articles selected for inclusion were those which examined fatigue and/or strategic napping among members of the general adult population or professional pilots. 3. Results Reflecting the severity of fatigue as a threat to health and safety, the volume of literature available on the subject is tremendous. The present review will provide conclusions from relevant publications regarding the effects of fatigue on pilots, as well as the benefits and potential risks of implementing strategic naps. Section 3.1 will focus on the cognitive and performance deficits due to fatigue which may be of greatest concern to aviators, and Section 3.2 will explain both the pervasiveness and potential sources of fatigue among pilots. Sections 3.3 and 3.4 will detail the results of studies testing the benefits of strategic napping among the general population and for pilots in particular, respectively. Finally, Section 3.5 will discuss recommendations for the safe and effective use of in-flight naps. 3.1. Cognitive and performance deficits due to fatigue A wide variety of cognitive and performance deficits may be evident during periods of sleep loss, including total sleep

deprivation, chronic sleep restriction (i.e., several consecutive days of insufficient sleep), and health conditions which may disrupt sleep throughout the night (e.g., obstructive sleep apnea). Of the types of sleep loss not related to health factors, chronic sleep restriction is more commonly experienced within the modern workforce yet the majority of the existing research has focused on total sleep deprivation. However, the impairments are similar for either type of sleep loss. For example, many studies have reported deteriorated competence on measures of attention and psychomotor performance (e.g., Haavisto et al., 2010; Killgore et al., 2009; Krueger, 1989; Tucker et al., 2010; Van Dongen et al., 2004) with most studies using the Psychomotor Vigilance Task (PVT, Dinges and Powell, 1985) as the primary means of assessment. Comparable deficits have also been observed for measures of divided attention (Jackson et al., 2011) in which participants were directed to respond to both visible and audible stimuli, with participants committing significantly more errors of omission and commission when sleep deprived than when they were well-rested. Further, fatigue has also been shown to negatively affect visual performance, with sleep loss resulting in deficits such as decreased binocular convergence (Quant, 1992) as well as increased visual neglect for centrally and peripherally presented stimuli (Kendall et al., 2006; Rogé et al., 2003). In particular, Rogé and colleagues reported that after a night of total sleep deprivation, participants in a 60-min driving simulator task exhibited a decreased sensitivity to changes in the peripheral environment in comparison to their performance when normally rested. Additionally, decrements in cognitive abilities have also been observed. For example, continuous wakefulness in excess of 20 h contributes to increased difficulty in forming new memories (Yoo et al., 2007), as well as poorer working memory for auditory (Tyagi et al., 2009) and visual stimuli (Smith et al., 2002). Working memory deficits may be especially critical because working memory capacity reflects the amount of information an individual can mentally manipulate at a given time and is thus strongly related to problem solving ability. Reflecting this relationship, fatigue researchers have observed significant performance deficits in problem solving ability (Linde and Bergström, 1992), reasoning abilities (May and Kline, 1987; Neri et al., 1992), and divergent thinking (Horne, 1988). Likewise, a deterioration of processing abilities has also been observed, such as impaired spatial processing (Neri et al., 1992) as well as decreased processing speeds, (Hardaway and Gregory, 2005; Sanders et al., 1982; Zuckerman et al., 2007). Detrimental physiological and psychological effects have also been observed in conjunction with fatigue due to sleep loss. These effects include an overall decrease in general health and well-being (Eriksen and Åkerstedt, 2006) as well as a significant increase in the levels of the stress hormone cortisol after both a night of total sleep deprivation (Leproult et al., 1997) and 6 consecutive nights of sleep restriction (Spiegel et al., 1999). Repeated sleep loss is also associated with an increased risk for diabetes (Spiegel et al., 2005) as well as cardiovascular complications and slower metabolism (Meerlo et al., 2008; Vgontzas et al., 2006). In addition to impaired performance, changes in behavior and subjective experience have also been observed in conjunction with sleep loss, such as an increased rate of psychiatric disorders (Meerlo et al., 2008), increased risk taking behavior (Killgore et al., 2006; Venkatraman et al., 2007) and decreased optimism and sociability (Haack and Mullington, 2005), as well as increased subjective sleepiness (Axelsson et al., 2008; Balkin et al., 2008; Tucker et al., 2010). Using flight simulators as well as monitoring performance during actual flights, other detrimental effects of fatigue have also been observed which may be of particular concern to pilots. For example, Russo et al. (2004) found that during a simulated 12-h overnight flight, pilots demonstrated increased visual neglect for stimuli both in the central and peripheral visual fields of awareness. Likewise,

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Petrilli et al. (2006) reported that during long-haul flights, pilots’ response times slowed significantly from the beginning of the flight to the end of the flight. These types of impairments may cause pilots to be unaware of alerts and dangers, while also impairing their ability to respond to such situations appropriately. Additional research has found that behavioral or affective changes are evident as a result of sleep loss, such that pilots may take unnecessary risks (Hardaway and Gregory, 2005; Neri et al., 1992) or become overly confused, stressed, or frustrated (Drury et al., 2012). Moreover, Spencer and Robertson (2002) determined that the occasional unexpected problems occurring during flight which led to an increase in perceived workload (i.e., hassles) significantly worsened subjective fatigue during the flight, which may further contribute to the behavioral changes observed. These negative affective states likely contribute to further deterioration of processing and decision making abilities, and thus may exacerbate any problems which might arise on the flight deck. 3.2. Sources and pervasiveness of fatigue in aviation Chronic fatigue has been identified as a widespread problem for the modern workforce (Chee and Chuah, 2008; Ohayon, 2012; Rajaratnam and Arendt, 2001). Professional pilots are likely among those who continue to work when fatigued, typically without incident, and thus may fail to recognize the inherent risks associated with such behavior (Wagenaar and Groeneweg, 1987), yet, the severity of the situation and the associated detrimental effects on performance appear to be underappreciated by the aviation community (Caldwell et al., 2009). For example, pilots are susceptible to microsleeps or brief, inadvertent naps while in the cockpit, with some unplanned naps lasting longer than 10 min (Graeber et al., 1990). Additionally, several surveys have found that pilots believe fatigue is a significant problem, impacting both their work performance as well as their personal time. For example, the majority of pilots surveyed reported increasing subjective sleepiness and pressure to sleep (Gregory et al., 2010; Gundel et al., 1995; Lowden and Åkerstedt, 1998; Petrie et al., 2004; Petrilli et al., 2006) during their duty period. Likewise, data collected from long-haul pilots indicated that nearly 66% experienced significant fatigue at least once a week that was associated with their work schedule, and more than 96% reported that fatigue had interfered with their usual social activities during the last month (Petrie et al., 2004). However, McKenney et al. (2000) reported that few of the international air carriers represented in their survey offered any type of training on fatigue mitigation techniques. Additionally, Taneja (2007) reported that 94% of the pilots surveyed felt that they needed up-to-date training on sleep hygiene and the effects of fatigue, as well as circadian rhythms. In response to these types of concerns, as well as information gained from mishaps such as that near Kirksville Regional Airport (Section 1), the NTSB released recommendations that fatigue management programs be mandated as part of a pilot’s initial and reoccurring training (Federal Aviation Administration, 2012). One of several factors which may contribute to this chronic fatigue among pilots is the extended flight duty period associated with long-haul, international flights. In particular, pilots of these flights are likely to have duty periods in excess of 8 h and thus are at the greatest risk for performance impairment due to fatigue (Rosekind et al., 1995). For example, a 9-h flight may involve a duty period of 12 h or more. Justifying concerns with regard to the potential dangers associated with these long-haul flights and extended duty periods, Goode (2003) examined the records of accidents in commercial aviation between 1978 and 1999 and found a significant relationship between flight duration and frequency of accidents. Specifically, his analyses revealed that 20% of the accidents attributed to human factors problems happened when the

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pilot had been on duty more than 10 h, and 5% happened when the pilot had been on duty more than 13 h, though these durations represented only 10% and 1% of the total flights, respectively. Additionally, as described in Section 3.2, pilots have reported increasing fatigue across the duration of the flight (c.f., Gundel et al., 1995; Petrilli et al., 2006), suggesting that the combination of time-ontask fatigue (Samel et al., 1997) and the strong drive to sleep after someone has been awake for a long period of time (i.e., homeostatic sleep pressure) may lead to a serious deterioration in the pilots’ alertness and performance. A second factor influencing the fatigue experienced by pilots may be the amount of sleep time lost due to duty periods beginning at either unusually early or late times (Roach et al., 2012; Rosekind et al., 1994; Samel et al., 2004; Spencer and Robertson, 1999a,b). Specifically, for every hour prior to 0900 that a duty period begins, pilots are estimated to lose approximately 15–30 min of sleep (Roach et al., 2012; Spencer and Montgomery, 1997). Moreover, if a pilot has several consecutive days of early starts, the detrimental effects of sleep loss are quickly compounded (Spencer and Robertson, 2002). However, many pilots report that they are unable to get to sleep any earlier to compensate for the early morning starts (Samel et al., 2004; Spencer and Montgomery, 1997). Consequently, pilots who are assigned to extended duty periods and long-haul flights beginning in the morning may be exhausted and significantly impaired during the approach and landing phases of flight if no fatigue countermeasure is implemented. A third factor which may increase feelings of fatigue is the disruption of the sleep/wake cycle experienced when rapidly changing time zones, commonly described as “jet lag” (Kandelaars et al., 2006). Caused by a discontinuity between the body’s internal clock and hormonal rhythms and the exogenous cues in the new environment (e.g., the individual is experiencing high circadian pressure to sleep even though the local time is 1300 h), jet lag can lead to increased difficulty in obtaining recovery sleep during layovers (Eriksen and Åkerstedt, 2006; Kandelaars et al., 2006; Lowden and Åkerstedt, 1998). Further, during the low-points in the circadian rhythm when sleep pressure is highest, often referred to as the circadian trough, performance also deteriorates in a manner similar to that observed during sleep loss (Schmidt et al., 2007). Thus, if a pilot is on a returning flight which departs at 1035 local time but his endogenous circadian clock indicates that it is 0235, the pilot will likely experience decreased alertness and increased fatigue. In addition to these schedule-related factors, there are some environmental factors which may increase both sleepiness and the risk of an aviation mishap. For example, technological advances have made it possible to automate much of the activity on the flight deck, which can reduce both the cognitive workload for the pilot as well as the number of crew members needed (Rosekind et al., 1995; Samel et al., 1997). “Glass cockpits”, flight decks in which status information is presented on electronic screens, have all but replaced traditional analog displays. Working in this type of monotonous environment, especially during long-haul flights, may lead to deterioration in vigilance, as well as increased boredom and complacency, and consequently increasing subjective sleepiness (Neri et al., 2002; Spencer and Robertson, 1999a,b; Stokes and Kite, 1994). The increased feelings of fatigue may be worsened by other factors, such as dim lighting on the flight deck (Graeber et al., 1990) as well as the absence of tactile feedback through the analog controls (Roach et al., 2012). Additionally, sudden stressful events during flight may exacerbate these feelings of subjective fatigue (Spencer and Robertson, 2002). These types of situational factors may be intensified by physiologic stressors such as hypoxia (reduced oxygenation of bodily tissue) or hypoxemia (reduced oxygenation of arterial blood). Reduced oxygenation is common among pilots flying at 8000 ft above sea level and the resulting symptoms are very similar to those associated with fatigue such as decreased

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vigilance and impaired judgment (Aerospace Medical Association, Aviation Safety Committee, Civil Aviation Subcommittee, 2008; Harding and Mills, 1983). Finally, the health of the pilot is also critical as any type of medical condition which interferes with sleep quality would consequently contribute to increased fatigue and reduced alertness while on duty. One disorder which is of the greatest concern due both to the effects on sleep as well as the prevalence in the U.S. is obstructive sleep apnea (OSA; Williamson et al., 2011). Affecting an estimated 20% of adults, OSA is described as total or partial blockage of the upper airway which forces the sleeper to work harder to breath and in many cases may cause the individual to wake up coughing or struggling to breathe (Panton et al., 1997; Shamsuzzaman et al., 2003). These sleep disruptions are consequently related to decreased sleep quality, greater daytime fatigue, and an increased risk for cardiovascular disease (Shamsuzzaman et al., 2003). Addressing the issue of OSA among commercial pilots, the Federal Aviation Administration Civil Aerospace Medical Institute (2010) released an informational pamphlet detailing risk factors, symptoms, and courses of treatment. The most widely advocated treatment is for the individual to lose weight as incidence rates are highly correlated with obesity (Panton et al., 1997), but physicians may also recommend use of a Continuous Positive Airway Pressure (CPAP) machine or dental appliances. At any given time, pilots are likely experiencing fatigue due to a combination of these factors. For example, a pilot suffering from OSA who consumes alcohol after a flight in which he crossed multiple time zones will experience very poor sleep quality while off-duty (Federal Aviation Administration Civil Aerospace Medical Institute, 2010). The increased sleepiness experienced the next day, combined with boredom often reported in the cockpit, would drastically increase the pilot’s likelihood of having microsleeps and unintentional naps or lapses in attention while in control of the aircraft. Likewise, a pilot’s judgment might be significantly impaired by the combined effects of sleep loss and mild hypoxia. Thus, collectively these factors greatly increase the likelihood that a pilot may experience severe performance impairments while on duty (Neri et al., 2002), placing the safety of the crew and any passengers in danger. Further the prevalence and associated risks of fatigue emphasize the fact that despite the technological advancements which have improved the efficiency of the modern flight deck, the needs of the human operator have not changed and adequate sleep remains a necessity. 3.3. Use of napping as a fatigue countermeasure Sleep is as much a physiologic necessity as food and water (Gregory et al., 2010), and as stated in the Introduction, nothing can compensate for losing sleep but the chance to recover it. Unfortunately, in many situations, a long sleep period may not always be possible, such as during long haul flights or military operations. Fatigue countermeasures such as prescription and over the counter analeptics (e.g., dextroamphetamine, modafinil, caffeine) may be an invaluable tool in maintaining alertness and performance during periods of extended wakefulness or restricted sleep. Although these alertness aids have been proven efficacious in the available literature (c.f., Caldwell et al., 2009; Killgore et al., 2008; Wesensten et al., 2005), they fail to address the actual source of fatigue – insufficient sleep. Consequently, these treatments serve only to maintain acceptable performance but do nothing to aid in the physiologic process of recovering from sustained wakefulness, sleep disrupted by medical conditions, or circadian influences. In these situations when fatigue is severe but obtaining a full sleep period is not possible, the implementation of strategic napping can be immensely beneficial, reducing both the drive to sleep and helping to restore

some of the cognitive performance deterioration associated with fatigue. Strategic naps of any duration are beneficial and can help to reverse performance deficits due to sleep loss or disruption (Driskell and Mullen, 2005), with naps as short as 10 min helping to reduce subjective sleepiness and improve neuropsychological performance (Tietzel and Lack, 2001). Although strategic naps may not reduce sleep pressure associated with the body’s internal clock, also known as circadian sleep drive, these shortened sleep periods can reduce the homeostatic sleep drive, which is the gradually increasing pressure to sleep the longer we have been awake (Caldwell et al., 2002). Naps may either be taken prophylactically, in anticipation of accumulated sleep loss, or operationally, in response to increasing sleepiness and impairment (Naitoh and Angus, 1987; Rosekind et al., 1995). Night shift workers often use prophylactic naps prior to their first shift of the week as a means of preparing for the upcoming night of sleep deprivation (Ficca et al., 2010). In their research, Bonnet et al. (1995) concluded that the efficacy of a 2-h prophylactic nap taken prior to 52 h of total sleep deprivation was comparable to repeated dosing of 150 mg of caffeine. Conversely, operational naps are especially valuable during periods of continuous or sustained wakefulness operations when alertness needs to be maintained over a long period of time (Angus et al., 1992). In particular, the authors examined the potential benefits of various behavioral fatigue countermeasures to determine which were effective during 52 h of total sleep deprivation during which participants worked on a military command center scenario. The results indicated that practices such as occasionally reducing cognitive workload and including brief bouts of strenuous exercises did not help to maintain performance, whereas a 2-h nap after either 40 or 46 h of total sleep deprivation prevented further performance deterioration. Numerous studies have examined the benefits of napping during a period of otherwise continuous wakefulness, with naps ranging in duration from 30 s to 2 h, and these benefits include maintaining cognitive performance as well as reducing objective and subjective sleepiness. In particular, performance on tasks assessing memory and recognition (Dinges et al., 1988; Mullaney et al., 1983), mental addition and subtraction (Bonnet et al., 1995; Dinges et al., 1988), logical reasoning (Bonnet et al., 1995), and stimulus-response compatibility (Asaoka et al., 2012) revealed superior performance over those who did not have a nap opportunity. Improved attention and vigilance as well as decreased reaction time have also been observed as a benefit of napping, for both auditory (Mullaney et al., 1983) and visual (Bonnet et al., 1995; Dinges et al., 1988; Vgontzas et al., 2007) stimuli. Additionally, sleep onset latency, assessed according to the Multiple Sleep Latency Test (MSLT) is also typically greater following a nap (Lumley et al., 1986; Vgontzas et al., 2007), indicating that naps shorter than 2 h can effectively reduce an individual’s objective sleep drive. Likewise, napping during periods of extended wakefulness also reduced subjective sleepiness (Tietzel and Lack, 2001; Vgontzas et al., 2007). 3.4. Napping in flight Scheduled naps while in flight may be of particular value to pilots (Eriksen et al., 2006; Roach et al., 2010; Rosekind et al., 1991), especially when considering the increased workload and potential hazards associated with the final approach. The final approach and landing is regarded as the most demanding phase of the flight, as evidenced in a survey of worldwide commercial airline accidents conducted by Boeing Commercial and Airplanes (2012). From 2002 to 2011, the greatest proportion of fatal accidents involving commercial jets occurred during the final approach and landing phases of the flight, indicating not only how critical this period is, but also the necessity of the pilot being alert and vigilant. However, as they

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prepare to descend, long-haul pilots have likely been awake and on duty for a long period of time (Roach et al., 2010), and thus are experiencing a great deal of pressure to sleep which is reflected in the increasing number of microsleeps and deterioration in attention (Rosekind et al., 1994; Sletten et al., 2005). As a result, a pilot’s risk of having inadvertent sleep periods and lapses in attention may be highest during the most demanding and dangerous phase of the flight. To aid in maintaining alertness during long-haul flights, augmented crews (i.e., one or more additional pilots on duty who can take over and allow members of the primary crew to rest) can allow pilots to rest in designated bunk facilities, a practice generally accepted by most if not all air carriers (McKenney et al., 2000). Conversely, for an un-augmented crew which does not include any relief personnel, cockpit napping or naps taken on the flight deck is also beneficial, though this practice is not permitted by all airlines. 3.4.1. Napping in rest facilities Most of the commercial aircraft designed for long-haul flights, such as the Boeing 747-400 and the Airbus A340, are equipped with rest facilities including twin bunks which are available to augmented crews (McKenney et al., 2000). These rest facilities are typically adjacent to the cockpit but separated by either a curtain or solid door. Rules for nap duration in these bunks vary by airline and by country, but in general the arrangement of the rest times are determined by the crew members, with regulations established on how long all crew members must be awake before the aircraft begins the final approach. Although naps taken in the bunks are not reported to be as restorative as sleep obtained in a hotel or at home (Pascoe et al., 1994; Roach et al., 2010; Sletten et al., 2005), these sleep opportunities are more beneficial than attempting to nap upright in a chair (Civil Aviation Administration, 2003; Eriksen et al., 2006), and any sleep opportunity is better than continuous wakefulness (Driskell and Mullen, 2005). Several studies have examined both the benefits of strategic naps taken in these rest facilities as well as the factors which might influence the quality of sleep. Although the predominant focus of this research is on examining matters of scheduling, additional factors may include details such as how much sleep the pilot had the previous day (Roach et al., 2011) or whether the pilot is from a culture in which mid-day breaks (i.e., siestas) are common (Holmes et al., 2012). One of the most commonly reported factors related to fatigue and an increased drive to sleep while on duty is which leg of the flight the pilot is currently flying, that is, whether it is the outbound or returning flight. Among commercial aviators, studies have commonly found that both subjective (Gundel et al., 1995; Lowden and Åkerstedt, 1998) and objective (e.g., Baker et al., 1992) fatigue are greater on the return flight than the outbound flight. For example, research from Eriksen and colleagues (Eriksen and Åkerstedt, 2006; Eriksen et al., 2006) examined total sleep time using bunk rest facilities for three-pilot crews whose duty period started in either Copenhagen or Stockholm and who flew to New York City or Washington DC, and then returned to their domicile. Their results indicated that the return flight was associated with significantly greater levels of fatigue as evidenced by greater total sleep time during the on-duty nap as well as higher estimate of subjective fatigue. This was especially surprising given that the average main sleep period prior to the return flight was nearly two hours longer than the sleep period prior to the outgoing flight, suggesting that pilots would be even better rested in preparation for the return flight. Additionally, the authors concluded that these strategic naps were beneficial because the pilots who had greater total sleep time also reported significantly greater subjective performance after the nap. Similarly, Holmes et al. (2012) reported that on ultra-long range flights (i.e., flight time greater than 16 h), the returning flight total sleep time was more than an hour greater than was observed on the outgoing flight. Further, prior to the

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in-flight sleep opportunity, subjective sleepiness was greater on the return flight than on the outbound flight. Other factors which may influence fatigue levels while on duty include circadian and homeostatic processes. Circadian rhythms refer to the body’s 24-h endogenous clock which naturally controls wake and sleep patterns as well as body temperature and the release of certain hormones. The circadian rhythm is largely responsible for the severe sleepiness experienced both early in the morning as well as in the mid-afternoon hours. Conversely, homeostatic sleep pressure is the body’s drive to sleep which gradually increases the longer someone is awake. In other words, an individual who has been awake for an extended period of time will likely have a greater homeostatic drive to sleep than someone who has just recently woken up. Examining the effect of these influences on sleep quality while on duty, Baker et al. (1992) compared napping in the evening (1830–2230 h) with a sleep period taken during the night (2300–0700 h) in a replica of the rest facilities on board a Boeing 747-400. As expected, participants experienced greater sleep efficiency during the overnight sleep than during the evening nap, suggesting strong circadian and homeostatic influences. However, somewhat opposing results were obtained by Eriksen and Åkerstedt (2006). These authors concluded that there were no differences with regard to in-flight total sleep time between morning flights and evening flights. However, supporting the notion of strong homeostatic pressures, these pilots did report significantly greater subjective fatigue for the evening flights rather than the morning flights. Also examining the effect of homeostatic sleep pressure on the quality of in-flight naps, Pascoe et al. (1994) surveyed pilots regarding the benefits of napping early or later in the flight. Survey results revealed that pilots believed naps obtained in the latter half of the flight were more restorative and of a better quality than naps taken earlier in the flight. This difference in subjective sleep quality may be attributable to increased sleepiness resulting from both fatigue due to time on task (i.e., time spent working in the monotonous cockpit environment described in Section 3.2) as well as increasing homeostatic sleep pressure, as those who took their nap later in the flight had been without sleep longer than those who napped earlier in the flight. Similar results were obtained by Signal et al. (2003), who further reported that naps taken later in flight were significantly longer and of better quality than naps taken earlier in the flight, even when controlling for total sleep time during the last main sleep period. However, Sletten et al. (2005) reported that pilots in their study slept better during the first half of the flight than during the second half of the flight. The authors attributed this difference not to a decreased sleep need later in the flight but rather to anxiety regarding the upcoming approach and landing phases of flight. 3.4.2. Napping on the flight deck For un-augmented crews, use of the on-board rest facilities is not available, so strategic naps may only be taken in the cockpit. The US Federal Aviation Administration (FAA) does not currently permit napping on the flight deck, citing safety concerns and a lack of evidence that the benefits outweigh the risks. However, both the Aerospace Medical Association (Federal Aviation Administration, 2012) as well as heads of foreign air carriers (McKenney et al., 2000) strongly advocate the practice, saying that these naps restore the crews’ alertness and vigilance, thus helping to ensure the safety of the flight. McKenney and colleagues report that for the countries and airlines which do allow cockpit naps, the specific rules governing the use of napping in the cockpit vary by country and by airline though there are some common guidelines. In particular, these naps are generally limited to a duration of 15–30 min and may only be taken during the cruise portion of the flight when workload and cognitive demand are lowest. Additionally, pilots are expected to

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notify flight attendants before napping to ensure that the attendant regularly checks on the alertness of the pilot who remains in control of the plane. Although the quality of sleep obtained in the cockpit will likely not be as good as naps taken in on-board rest facilities due to noise and activity as well as not being able to fully recline, pilots report that cockpit naps do help to reduce fatigue and restore vitality (Ficca et al., 2010). To address the FAA’s safety concerns, scientists with the NASA Ames Research Center studied the effectiveness of strategic naps taken in the cockpit to maintain performance and alertness during long-haul flights (Rosekind et al., 1994). Pilots assigned to the Rest Group were allowed 40 min to nap in their seats during the cruising phase of the flight, with this rest period ending at least 60 min before descent, whereas pilots in the No-Rest Group had no sleep opportunity and continued with their normal assignments during the corresponding 40-min time frame. Physiologic and performance data were collected both before and after this 40min block, during which participants either rested or continued with their normal work. The data collected included polysomnographic (PSG) data to objectively assess alertness throughout the flight and sleep quality during the nap, as well performance on the handheld Psychomotor Vigilance Task which evaluates sustained attention as well as reaction time. On average, sleep onset latency for pilots in the Rest Group was less than 6 min, suggesting that these pilots were experiencing pathological levels of daytime sleepiness (Carskadon et al., 1986) while in control of the aircraft. These pilots napped for approximately 26 min, with more deep sleep being observed during nighttime flights than daytime flights. In comparison to pilots who did not nap, pilots in the Rest Group demonstrated significantly better response times as well as significantly fewer lapses in attention during the final approach. Examining alertness and napping among pilots of multi-leg trips, Spencer and Robertson (1999b) found that on long-haul legs of the flights they studied, many of the pilots took naps in the cockpit, with an average duration of 47 min. Likewise, a survey of the fatigue countermeasures implemented and favored by international pilots found that more than half of the respondents had utilized cockpit naps in the previous year (Petrie et al., 2004). These pilots who reported taking cockpit naps also indicated that they experienced less overall subjective fatigue and greater vitality than did pilots who did not nap in the cockpit. Thus, as determined by Driskell and Mullen (2005), naps of any duration, and even in less than ideal conditions, may be beneficial in maintaining alertness and improving safety. 3.4.3. Limitations and contraindications to napping on duty As stated in Sections 3.4.1 and 3.4.2, studies have found that naps taken while on board the aircraft, either in the rest facilities or in the cockpit, are not as restorative as sleep obtained at home. There are a number of factors which may disrupt sleep taken while onboard an aircraft, such as noise coming from the plane itself or nearby activities, turbulence, excessive lighting, and atmospheric conditions such as temperature and humidity (Civil Aviation Administration, 2003; Baker et al., 1992; Pascoe et al., 1994; Urponen et al., 1988). The survey of pilots conducted by Pascoe et al. (1994) revealed that nearly 66% of pilots had taken additional measures to improve their sleep quality, with most of them using ear plugs and/or sleep masks. Additionally, some pilots even reported that feelings of anxiety about not being in control of the plane kept them from being able to nap (Baker et al., 1992; Civil Aviation Administration, 2003; McKenney et al., 2000). Another factor which may influence the efficacy of a strategic nap is the circadian timing of the nap (Ficca et al., 2010; Pascoe et al., 1994; Sletten et al., 2005; Tietzel and Lack, 2001). Several studies have reported that naps taken during the times when the

circadian drive to sleep is highest (i.e., circadian troughs) are more beneficial and have shorter sleep onset latencies than naps taken at other times during the day (Lumley et al., 1986; Naitoh et al., 1982). For example, pilots napping on the flight deck during daytime flight experienced more light sleep (i.e., more Stages 1 and 2 sleep) while they experienced more deep, slow wave sleep during the nighttime flights (Graeber et al., 1990). However, other authors have concluded that the circadian timing of the nap has no influence on the benefits of the nap (Driskell and Mullen, 2005) or on the duration of the nap (Eriksen and Åkerstedt, 2006). In particular, Eriksen and Åkerstedt reported that there was no difference in napping patterns between pilots of flights leaving in the morning and those on flights leaving in the evening. In addition to environmental and situational factors which may interfere with both the quality and quantity of sleep, concerns over the detrimental effects of sleep inertia may also discourage many from using these opportunities to nap. Sleep inertia is the period of cognitive and mood impairment, as well as hypo-vigilance, experienced immediately upon awakening (Dinges, 1990), and during this time, performance is even more impaired than it was prior to sleeping (Ferrara and De Gennaro, 2000; Wilkinson and Stretton, 1970). Some of these impairments include poorer decision making ability (Bruck and Pisani, 1999) and impairment in solving simple math problems (Wilkinson and Stretton, 1970), as well as slower reaction times on a recognition task (Achermann et al., 1995). Most research indicates that the detrimental effects of sleep inertia on performance last approximately 30–60 min (e.g., Bruck and Pisani, 1999; Ferrara et al., 2000), though the impairments have been observed up to 2 h after awakening (Jewett et al., 1999). As a real-world example of the dangers associated with sleep inertia, Armentrout et al. (2006) partially attributed the near loss of a USAF C-5 Galaxy to sleep inertia. All three members of the crew were significantly sleep deprived, and two members of the crew were awakened just minutes before beginning the descent. Likely owing to the aircrew’s exhaustion as well as the severe impairment due to sleep inertia, the report listed several mistakes such as misinterpreting instruments, not responding appropriately to system malfunctions, deteriorated situational awareness, and a breakdown in communication. At nearly 5000 feet above sea level (ASL), the engine stalled and the aircraft descended rapidly, yet the aircrew managed to recover control at less than 800 feet ASL and land safely with minimal damage. This extreme situation clearly demonstrates the necessity of scheduling enough time after awakening to fully recover from sleep inertia, thus avoiding the dangers which were encountered in this instance. Several factors are thought to influence the severity of sleep inertia experienced upon awakening. One of these factors is how long the individual was awake prior to this sleep opportunity (Dinges, 1990; Lumley et al., 1986; Tietzel and Lack, 2001). For example, Dinges et al. (1985) found that participants who had been awake continuously for more than 36 h prior to a 2-h nap experienced a significantly greater performance deficit upon awakening than did participants who had been awake for less than 30 h prior to the nap. A second factor which may influence the severity of sleep inertia is the circadian timing of the nap. Specifically, pilots waking up from a nap during a circadian trough will likely experience more severe sleep inertia due to the combination of the decreased alertness associated with the circadian trough and that associated with sleep inertia (Dinges et al., 1985; Lumley et al., 1986). Finally, the sleep stage from which the person is awakened may also influence the severity of sleep inertia, such that awakening from deep or slow wave sleep is commonly followed by a more severe impairment than awakening from light sleep (Ferrara and De Gennaro, 2000; Vgontzas et al., 2006). Despite these concerns, one survey found that most pilots felt the benefits of sleep opportunities while on duty and the subsequent decrease of

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performance deficits associated with fatigue greatly out-weighed the risks associated with sleep inertia (Gregory et al., 2010). 3.5. Recommendations for pilots regarding fatigue and napping Due to the chronic nature of fatigue in modern society, researchers have widely advocated the use of training programs which will help instruct individuals on matters such as proper sleep hygiene (Caldwell, 1997; Hardaway and Gregory, 2005). For example, these training programs explain how different conditions or activities can negatively influence sleep quality, such as sleeping in a room that is too warm or has windows without sufficient coverings to block out exterior light, as well as consuming alcohol, caffeine, and high-fat meals shortly before bed. Additionally, Hardaway and Gregory reported that pilots benefited significantly from receiving guidelines on the best times to sleep during layovers, such that pilots who received the training reported better sleep quality and lower fatigue than did pilots who did not receive the training. Unfortunately, as explained in Section 3.2, few of the commercial pilots surveyed reported receiving any type of training for sleep management and use of fatigue countermeasures by their air carrier (McKenney et al., 2000). Thus, even though ideal schedules may not be available to pilots, most would likely benefit greatly from guidance on how to make the most of their sleep opportunities. In this same field of literature, recommendations have also been made regarding techniques to decrease the severity of fatigue among pilots, and to improve both the efficacy and the safety of strategic naps. One such area which may influence fatigue is the size of a crew on a long-haul flight. For example, Eriksen et al. (2006) reported that most pilots felt augmented, 3-man crews were more beneficial toward maintaining alertness than un-augmented crews of 2 pilots. Their survey indicated that members of unaugmented crews experienced greater subjective sleepiness, as well as increased boredom and more depressed mood during the flight. Due to concerns regarding the lack of stimulation from having fewer crew members with whom to interact, as well as reduced sleep opportunities experienced with un-augmented crews, suggestions have been made regarding the ideal crew sizes for different flight situations. For example, Spencer and Robertson (1999a,b) recommend only augmented crews should be used for flights longer than 10 h. Further, Samel et al. (1997) advocated that crew size should be determined both by the flight duty period and by the circadian timing of the flight. Specifically, they suggested that for daytime flights with two pilots, flight duty periods should be no longer than 12 h, whereas flight duty periods during evening or early morning hours should be limited to 10 h. Similarly, Spencer and Robertson (1999b) suggested that un-augmented crews would be sufficient for long-haul flights beginning in the morning (i.e., between 0400 and 1100 h), whereas augmented crews should be utilized for flight duty periods beginning in the late afternoon or evening hours. The latest guidance from the Federal Aviation Administration (2012) addresses some of these concerns, mandating that duty periods in excess of 14 h must have an augmented crew. Further, flight duty periods are restricted for both un-augmented and augmented crews based on the local time at which the duty begins. Even when implementing these scheduling suggestions, overwhelming feelings of fatigue will likely still lead pilots to take naps in order to maintain alertness and performance. The available literature also provides suggestions on what steps may improve the efficacy of strategic naps, such as taking one long nap rather than several shorter naps over the duration of the flight (Pascoe et al., 1994). Further, Roach et al. (2010) recommend steps which will reduce disruptions during any sleep opportunity, such as training pilots how to relax during their opportunity to sleep, as well as environmental controls which may reduce sensory disturbances, such

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as loud, irregular noises or sudden changes in lighting. Specifically, using ear plugs and sleep masks is an effective means of reducing or eliminating environmental disruptions. As explained in Section 3.4.3, sleep inertia is one of the greatest contraindications to using in-flight naps to maintain performance, but the related risks may be easily mitigated. One of the most effective ways of reducing the dangers associated with sleep inertia after a nap is to allow sufficient time for sleep inertia to subside before returning to duty. As demonstrated in the description of a near-mishap involving a USAF C-5 (Armentrout et al., 2006), awakening immediately prior to or during the descent is dangerous as performance during the first 30–60 min after awakening is significantly impaired (Dinges et al., 1985; Ferrara and De Gennaro, 2000). Although most air carriers state that all pilots must be awakened 20–30 min before beginning the descent (Civil Aviation Administration, 2003; McKenney et al., 2000), many of the pilots surveyed by McKenney and colleagues stated that being awakened from a nap 30 min before the beginning of descent was insufficient time to recover from sleep inertia. These types of responses indicated that naps should not be discouraged due to concerns regarding sleep inertia, but crews must ensure that sufficient recovery time will be available before a pilot must return to the controls (Takahashi, 2003). Several steps have also been suggested which may reduce the severity of sleep inertia experienced upon awakening from the nap. For example, Dinges et al. (1985, 1987) explained that the severity of sleep inertia may partially depend on how long the person had been awake prior to the nap, especially if the person had been awake for more than 36 h. Thus, if a pilot is anticipating an extended flight duty period, he or she should try to obtain a nap early in this period of continuous wakefulness, both to reduce the severity of sleep inertia and to better maintain performance during the remainder of the duty period. Additionally, the circadian timing of the nap may also influence the severity of sleep inertia. Consequently, pilots will likely benefit most from naps started near the end of their circadian trough, as this natural dip in alertness may help to reduce sleep onset latency, and upon awakening they may be approaching a circadian peak which will help to boost alertness in addition to the benefit derived from the nap.

4. Conclusions Fatigue due to sleep loss or poor sleep quality has become an overwhelming problem in modern society, having detrimental effects not just on physical and emotional well-being, but also on measures of performance such as reaction time, alertness, decision making, and cognitive processing. Moreover, some deficits have been recognized which may be of particular concern for pilots, such as visual neglect of the central and peripheral visual fields of awareness, as well as increased susceptibility to stress and confusion. As a whole, these types of impairments may not only impair a pilot’s ability to recognize a problem in time to take corrective actions, but they could also decrease his or her adeptness in determining and implementing the appropriate solution. Thus it is not surprising that fatigue is described as one of the greatest threats to aviation safety, having been identified as a contributing or causal factor in numerous accidents. To combat the decreased alertness and performance due to fatigue, there are several countermeasures available which may help to alleviate some of these problems. The most beneficial of these countermeasures is the use of strategic napping. Specifically, these naps not only help to maintain and restore alertness and performance, but also help to reduce the accumulated sleep debt, and these benefits may be evident in naps

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as short as 10 min. Often, pilots may be able to nap during the cruising phase of long-haul flights either in the cockpit or in the plane’s onboard rest facilities. Although the duration and quality of sleep during these naps are not typically as good as full sleep periods taken at home, research indicates that strategic naps can help to increase subjective feelings of vitality, as well as objective performance and alertness. Although there are some risks associated with this countermeasure, such as the performance impairments due to sleep inertia upon awakening, both researchers and pilots agree that when scheduled properly, strategic naps are extremely beneficial both in maintaining the health and well-being of the pilot as well as ensuring the safety of the flight. Although existing research has well demonstrated the value of strategic naps as a fatigue countermeasure, further studies would be beneficial to examine particular performance decrements and the subsequent improvements among pilots who nap on duty. For example, the in-flight study conducted by Rosekind et al. (1994) used the Psychomotor Vigilance Task (PVT) and concluded that the 45-min nap opportunity resulted in superior alertness and sustained attention during the approach and landing phases of the flight over pilots who did not nap. Likewise, other researchers who have examined the benefits of on-duty napping have focused primarily on self-reports (Ficca et al., 2010) or assessing attention and vigilance (Petrie et al., 2004; Spencer and Robertson, 1999a,b), but little has been done to identify improvements on other measures relevant to flight safety. Specifically, by evaluating pilots either during flight or while in a high-fidelity flight simulator, researchers may be able to determine how much on-duty strategic naps may improve performance on other measures such as visual scanning, working memory, and judgment and decision making. Research such as this will help to determine the full range of skills relevant to pilot and aviation safety which may be restored to rested levels through the use of strategic napping. Acknowledgement The author would like to thank Dr. Lynn Caldwell for her helpful comments and guidance on this manuscript. References Achermann, P., Werth, E., Dijk, D.J., Borbély, A.A., 1995. Time course of sleep inertia after nighttime and daytime sleep episodes. Archives Italiennes de Biologie 134 (1), 109–119. Aerospace Medical Association, Aviation Safety Committee, Civil Aviation Subcommittee, 2008. Cabin cruising altitudes for regular transport aircraft. Aviation, Space, and Environmental Medicine 79 (4), 433–439. Angus, R.G., Pigeau, R.A., Heslegrave, R.J., 1992. Sustained-operations studies: from the field to the laboratory. In: Stampi, C. (Ed.), Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep, Vol. 217–241. Birkhauser, Boston. Armentrout, J.J., Holland, D.A., O’Toole, K.J., Ercoline, W.R., 2006. Fatigue and related human factors in the near crash of a large military aircraft. Aviation, Space, and Environmental Medicine 77 (9), 963–970. Asaoka, S., Fukuda, K., Murphy, T.I., Abe, T., Inoue, Y., 2012. The effects of a nighttime nap on the error-monitoring functions during extended wakefulness. Sleep 35 (6), 871–878. Avers, K., Johnson, W.B., 2011. A review of federal aviation administration research: transitioning scientific results to the aviation industry. Aviation Psychology and Applied Human Factors 1 (2), 87–98. Aviation Safety Network, n.d., 2013. Accident Descriptions, http://aviation-safety.net/database/dblist.php?Event=FCR (retrieved 30.04.13). Axelsson, J., Kecklund, G., Åkerstedt, T., Donofrio, P., Lekander, M., Ingre, M., 2008. Sleepiness and performance in response to repeated sleep restriction and subsequent recovery during semi-laboratory conditions. Chronobiology International 25 (2&3), 297–308. Baker B., Pascoe P.A., Rogers A.S., The quality of sleep in aircrew rest quarters: laboratory and preliminary inflight studies. Royal Air Force IAM Report No. 733, 1992. Balkin, T.J., Rupp, T., Picchioni, D., Wesensten, N.J., 2008. Sleep loss and sleepiness: current issues. Chest 134 (3), 653–660. Banks, S., Dinges, D.F., 2007. Behavioral and physiological consequences of sleep restriction. Journal of Clinical Sleep Medicine 3 (5), 519–528.

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