Sleep Loss and Performance
Sleep loss is a fact of life for most military personnel. In garrison or training environments, soldiers, sailors, airmen, and marines are expected to get up early, put in long hours, and often work late into the evening studying, training, maintaining equipment, or performing additional duties. This problem is often compounded during deployments, where soldiers may need to change time zones, operate for extended periods of time, and obtain sleep in dangerous or other inhospitable environments.
While society as a whole has been struggling with the problem of reduced sleep, service members face a number of challenges to their sleep that typically exceed those of the civilian community. Perhaps the most pervasive problem has been the long-held misperception among military personnel and their leadership that needing sleep is just a sign of laziness or weakness. As will be discussed in this chapter, nothing is further from the truth. Scientific evidence suggests that sleep is a vital contributor to warfighter performance. With the current complexities induced by asymmetric warfare, close contact urban environments, and network-centric operations, lack of sleep can degrade combat effectiveness as much or more than nearly any other element of resupply (Wesensten & Balkin, 2010). Rather than being a sign of weakness, obtaining adequate (p. 242) sleep can actually be a force multiplier and can enhance military capabilities. The present chapter provides an overview of the effects of sleep loss on performance and some methods for managing sleep and sustaining performance when optimal sleep cannot be obtained.
Alertness and Vigilance
Total Sleep Deprivation
Many studies have looked at the effects of going without sleep for one night, while a handful have studied the effects of prolonged wakefulness up to 3 or 4 days at a time. Interestingly, acute total sleep deprivation up to 88 hours has not been associated with adverse health consequences. It should come as no surprise, however, that the primary effect of prolonged sleep deprivation is a severely degraded capacity to remain alert, focused, and responsive to the environment, and an overpowering propensity to fall asleep despite strong motivation to remain awake. Military sleep-deprivation research has shown that alertness and vigilance performance remains relatively stable for about the first day of continuous wakefulness (about 16 to 18 hours awake). However, performance begins to severely degrade as wakefulness is extended beyond the normal bedtime (i.e., around midnight) and continues to decline throughout the nighttime and early morning hours, hitting a low around 0800. However, even with no sleep, normal circadian body rhythms will automatically help restore some modest level of alertness temporarily during the daylight hours, but reaction time performance will severely degrade again as nighttime approaches if sleep is not obtained (Wesensten, Killgore, & Balkin, 2005).
Not only does sleep deprivation produce sluggishness in general reaction time, it also leads to an increased propensity toward lapses in attention. A lapse is a period of nonresponsiveness to a stimulus, usually lasting half a second or longer. Some lapses may last several seconds, comprising a period when the individual is oblivious to incoming stimuli and lacks immediate situational awareness. These types of lapses are almost nonexistent in well-rested individuals, but become much more prevalent as the duration of sleep deprivation is increased. In fact, during the early morning hours of a second night of sleep deprivation, the probability of experiencing a lapse is about 1,000% greater than immediately after a normal night of sleep (Wesensten & Balkin, 2010). While a brief period of nonresponsiveness lasting a half-second or so may not seem like much, it could be the critical difference in shooting or being shot, deciding between friend or foe, failing to notice an improvised explosive device (IED), or simply having a motor vehicle accident. As a rule of thumb, it has been suggested that general alertness and vigilance performance can be expected to degrade by approximately 25% for each 24-hour period of sustained wakefulness (Belenky et al., 1994).
Partial Sleep Restriction
Continuous or sustained operations often preclude the ability to obtain a full night of sleep due to the exigencies of the mission. Over the past century, military leaders have often espoused the blatantly wrong assertion that soldiers can function effectively on 4 hours of sleep per day. While it is true that military personnel can continue to perform physically at a modest level for days or weeks with only limited regular sleep, scientific data suggest that alertness and vigilance capacities, which are critical for operational success in modern operational environments, are severely degraded by chronic sleep restriction. Figure 49.1 shows the reaction time performance for groups of participants given either 9, 7, 5, or 3 hours of time in bed to sleep each night over a weeklong stay in the laboratory. With each 2-hour reduction in nightly sleep, the ability to sustain mental focus and alertness was additionally degraded. Furthermore, those obtaining less sleep declined at a faster rate than those obtaining more sleep (Belenky et al., 2003).
Figure 49.1 also shows that even when the sleep-restricted participants were allowed three days of recovery sleep at end of the study (8 hours in bed per night), their alertness and vigilance performance never returned to prior baseline levels, suggesting that it may not be (p. 243) possible to fully “sleep it off” on the weekend following a period of chronically restricted sleep. It may take up to a week or more of normal sleep for performance to be restored to baseline levels after extended sleep restriction.
Finally, a recent survey of combat soldiers in Iraq (MHAT V) showed that when the amount of nightly sleep was cut by just 1 hour per night, personnel reported significantly greater difficulties handling the stresses of their jobs. Similarly, compared to soldiers reporting 7 to 8 hours of sleep per night, those getting only 4 hours of sleep per night were 250% more likely to report that they had made a mistake or had an accident that adversely affected the mission.
Sleep loss has severe effects on mood and emotional functioning. Military personnel who have been deprived of sleep for even a single night show significantly increased ratings of negative mood state. Furthermore, after two nights of sleep deprivation, soldiers showed significant declines in emotional intelligence and coping capacities, including reduced empathy for others, loss of self-esteem, and degraded appreciation of interpersonal dynamics (Killgore, 2010). Without sleep, soldiers showed poor frustration tolerance and increased symptoms associated with depression, anxiety, paranoia, and somatic complaints.
These impairments in emotional functioning may have a number of consequences in stressful military environments when unit cohesion and cooperation are critical. For example, a recent mental health survey of soldiers in combat (MHAT V) reported that rates of mental health problems nearly doubled for every 2 hours of chronically reduced sleep. Furthermore, sleep deprivation has been shown to reduce team performance in military settings (Baranski et al., 2007). Finally, sleep-deprived soldiers appear to be slower to make difficult, highly emotionally charged moral judgments compared to their rested performances, and were more likely to make judgments that violated their typical moral beliefs once sleep deprived (Killgore, 2010). In sum, sleepy soldiers are more irritable, more willing to compromise moral positions, less empathic, less cooperative and team-focused, and less able to cope with the stresses of combat than when normally rested.
Learning and Memory
The ability to encode and retain new information appears to be highly dependent on sleep. Evidence suggests that sleep is necessary before learning in order to prepare the brain to effectively encode information and is also necessary following learning in order to consolidate and integrate information into existing knowledge structures (Diekelmann & Born, 2010). Lack of sleep can have a modestly impairing effect on the ability to encode new semantic information (e.g., warning orders, commander’s intent, critical enemy position information, etc.), but appears to have a particularly impairing effect on temporal memory, or the order and timing for which specific events occurred. Thus, without sleep, military personnel may have some difficulty learning new information during training or assimilating information from briefings. They may also have greater difficulty recalling the order of specific events, which could be critical in tactical situations.
Recent evidence also suggests that sleep loss has a differential effect on various types of memory, with its greatest effects on some aspects of emotional memory. Specifically, sleep deprivation impairs subsequent recall of positive and neutral stimuli, but has no significant effect on recall of negative stimuli. While more research is necessary in this area, such preliminary findings raise the possibility that sleep deprivation during stressful combat settings could bias later recall toward the retention of negative and traumatic experiences over positive ones, potentially exacerbating posttraumatic stress or other adjustment problems.
Executive functions include a diverse set of cognitive capacities that are involved in the (p. 244) control and coordination of willful action to achieve future goals. These capacities include the ability to direct attention and cognitive resources, maintain information in immediate working memory, ignore irrelevant information, think flexibly, shift mental focus, form abstract concepts, and plan and sequence multiple steps. Considerable evidence now suggests that sleep deprivation impairs some, but not all, of these various capacities (Killgore, 2010).
Other than the previously described deficits in simple alertness and vigilance, current evidence suggests that sleep loss has only minimal if any significant effects on working memory capacity, logical deductive reasoning, reading comprehension, or nonverbal problem solving. In contrast, sleep deprivation appears to significantly impair divergent and innovative thought processes. Without adequate sleep, military personnel are likely to show deficits in the ability to think flexibly and creatively, plan ahead, prioritize information, detect errors and make appropriate corrections, and update courses of action when new information becomes available (Wesensten & Balkin, 2010).
Other evidence also suggests that sleep-deprived individuals tend to show greater risk-taking on behavioral tasks but deny such riskiness when queried on self-report measures, suggesting that self-awareness and judgment become impaired with sleep loss (Killgore, 2010). Consequently, when sleep deprived, a service member or military leader may begin to take greater risks but be unaware of this change. These skills and judgment capacities are often those that are particularly necessary for military commanders, who must maintain situational awareness, approach problems creatively, and adapt strategies and tactics to outmaneuver and defeat the enemy. Thus, the importance of obtaining adequate sleep for military leaders and decision-makers cannot be overemphasized. A sleep-deprived commander is at risk of mission failure due to an inability to think flexibly and adapt to a rapidly changing set of contingencies.
Individuals differ significantly in their biological need for sleep. Some people can function well on only a few hours of sleep per night, while others are likely to show deficits when obtaining less than 8 or 9 hours. On the whole, however, the majority of people begin to show some performance degradation when sleep is reduced below about 7 hours per 24-hour period. To maximize combat effectiveness, commanders should endeavor to provide 7–8 hours of sleep to their personnel within each 24-hour period (Wesensten & Balkin, 2010). Of course, mission requirements and the exigencies of combat will dictate how closely this suggestion can be followed. However, sleep should be considered on par with other vital components of resupply, such as food, water, fuel, and ammunition (Wesensten & Balkin, 2010). Military leaders need to protect the sleep opportunities of their personnel just as vigorously as they would protect these other critical elements.
Environments for sleep should be protected from noise, commotion, and light, and be kept cool and dry. Sleep facilities should be segregated to maintain a quiet and undisturbed area for those sleeping following their shifts. Finally, sleep is usually most efficient when obtained in a single session and at a time that is consistent with the downswing of the circadian rhythm of alertness (i.e., during nighttime hours—usually between 2300 and 0700). However, as long as 7–8 hours of sleep are obtained per 24-hour period, this sleep time may be broken into shorter periods (e.g., two 4-hour sleep periods). It is also important to be aware that most people experience a period of about 20 minutes of “sleep inertia” (i.e., postsleep mental sluggishness) upon awakening from a sleep episode lasting more than 20–30 minutes.
Exposure to bright light, particularly sunlight in the morning hours, is critical to entraining the circadian rhythm of alertness and sleep. (p. 245) When traveling across time zones, personnel should attempt to reset their circadian day via exposure to bright light in the morning of the new time zone as soon as possible and limit exposure to light during the evening to maximize the speed of adjustment of the new sleep schedule. When traveling eastward across time zones, most individuals will find that it takes approximately 1 day of adjustment for each time zone crossed before the circadian rhythm of alertness and sleep cycle catch up (e.g., crossing 5 time zones will take approximately 5 days before the sleep rhythm is normalized).
Some individuals are excessively sensitive to caffeine and should avoid this stimulant. However, when used appropriately, caffeine can be an effective temporary countermeasure to sleep loss for most individuals. Caffeine is present in many foods and beverages, but is most commonly found in coffee (approximately 100 mg per 8 oz. cup), tea, soda, and energy drinks. The effects of caffeine on alertness are only noticeable for about 3 to 6 hours after consumption for most people, but it may cause sleep disruption or sleep onset insomnia for up to about 6 or more hours. Small to moderate repeated doses of caffeine are most effective for sustaining alertness and vigilance during periods of extended wakefulness (Kamimori, Johnson, Thorne, & Belenky, 2005). For overnight operations, 200 mg (i.e., about 1 to 2 cups of coffee) every 2 hours has been shown to be effective at sustaining basic alertness and vigilance. The effectiveness of caffeine can be enhanced by a brief 15-minute nap. For example, if excessively sleepy, a soldier can consume one cup of coffee, set an alarm, and take a brief 15-minute nap. Upon awakening, the combined effects of the nap and caffeine will usually be effective at sustaining alertness for several hours.
Military psychologists must be aware of the effects of sleep loss on cognition, mood, and judgment in order to provide commanders with relevant information and advice on sleep management and in the assessment of unit readiness. In clinical settings, military psychologists need to consider the role that sleep loss may have in psychiatric presentation, including its effects on mood, personality functioning, and judgment.
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