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5 Sleep Disruption and Fatigue Performance following abrupt workload transitions during duty cycles of extended duration is markedly affected by three factors related to the daily cycle of sleep and wakefulness. When called on to work and sleep at irregular intervals for prolonged periods of time, these three factors can interact in an additive manner to create major impairments in performance. The first factor is circadian rhythmicity, which produces a near-24-hour internally driven cycle in alertness and performance. The second is sleep deprivation, the effects of which are evident after only a few hours of lost sleep. The third is sleep inertia, which results in impaired performance immediately on awakening from sleep. These three factors combine to make it exceedingly difficult to work effectively or safely for prolonged periods of time without a carefully planned strategy of countermeasures designed to address each factor. The purpose of this chapter is to review current understanding of each of these factors and their potential relevance to performance during workload transitions. CIRCADIAN EFFECTS ON PROLONGED PERFORMANCE It has long been recognized that psychomotor and cognitive perfor- mance on a variety of tasks in round-the-clock operations is at its worst during nighttime hours, generally reaching a nadir just before dawn (Bjerner et al., 1955~. In fact, the intrinsic nature of such daily rhythms was first demonstrated 260 years ago, when a French astronomer proved that the daily rhythm in leaf movement persisted even if a plant were kept in total 122
SLEEP DISRUPTION AND FATIGUE 123 darkness throughout night and day (deMairan, 17291. More than a century and a half ago, Duhamel discovered that these self-sustained oscillations persisted with a period that was close to (i.e., circa) but generally not ex- actly one day (hence our word circadian) when an organism was isolated from the periodic environmental light-dark cycle (DeCandolle, 1832~. However, the manner by which the light-dark cycle synchronized an endogenous cir- cadian rhythm with a non-24-hour period to the 24-hour day was not dis- covered until 30 years ago (DeCoursey, 1960; Hastings and Sweeney, 19583. At that time, it was found that brief exposures to light could induce phase shifts in endogenous circadian rhythms, and that the amount and direction of such shifts was dependent on the timing of the initial light exposure. The universality of this property of the circadian timing system was extended to humans only within the past three years (Czeisler et al., 1989~. Furthermore, the neurophysiologic basis for the generation of circadian rhythms in mammals was first uncovered less than 20 years ago. Ablation studies performed independently in two different laboratories indicated that bilateral destruction of a pair of hypothalamic neuronal clusters located on either side of the anterior tip of the third ventricle resulted in a loss of both endocrine and behavioral circadian rhythms (Moore and Eichler, 1972; Stephan and Zucker, 1972~. It was further demonstrated that these central nervous system structures, the suprachiasmatic nuclei (SCN) of the hypothalamus, received a direct, monosynaptic input from the retina via the retinohypothalamic tract (RHT) (Moore and Lenn, 19724. These initial studies, which sug- gested that the SCN might serve as a light-sensitive pacemaker of the mam- malian circadian timing system, have subsequently been supported by a variety of experimental manipulations. Multiunit recordings demonstrated a prominent circadian rhythm in the firing rate of SCN neurons, along with those in a number of other brain structures (Inouye and Kawamura, 19821. However, when the SCN were isolated from the rest of the brain via knife cuts, a circadian rhythm in firing rate was found only within the SCN (Inouye and Kawamura, 1979~. It would thus appear that a periodic signal emanating from the SCN drives a prominent circadian variation in central nervous system activation, as reflected by neuronal firing rate. Yet, surpris- ingly, neurophysiologic studies involving microinjection of tetrodotoxin have demonstrated that the firing of SCN neurons is not required for maintenance of the SCN's timekeeping function (Schwartz et al., 19871. Two different experimental paradigms have unambiguously established the link between this paired central nervous system structure and behavior. First, direct electrical stimulation of the mammalian SCN induces phase shifts in the behavioral rest-activity cycle comparable to those induced by brief exposures to light (Rusak and Groos, 19821. Second, transplantation of fetal SCN tissue rapidly restores rhythmicity of the behavioral rest-activ- ity cycle in SCN-lesioned animals (Drucker-Colin et al., 1984; Lehman et
24 WORKLOAD TRANSITION al., 1987; Sawaki et al., 19845. In fact, SCN transplantation from a mutant animal with an abnormally short intrinsic circadian period into a wild type animal whose own SCN had previously been destroyed results in an animal with a behavioral rest-activity cycle period comparable to that of the mutant (Ralph et al., 19903. It has thus been demonstrated that the genotype of the SCN determines the expression of this behavioral phenotype. The links between the SCN and human behavior are, of course, less definitively established. However, both the SCN and the RHT have been identified in the human brain (Hofman et al., 1988; Lydic et al., 1980; Sadun et al., 1984~. More than 60 years ago it was recognized that damage to the anterior tip of the hypothalamus resulted in abnormalities of the circadian sleep-wake cycle, leading to the suggestion that the neural center responsible for the timing of sleep might be located in that region (Fulton and Bailey, 1929~. Yet the first demonstration that human circadian rhythms persist in the absence of environmental time cues occurred only 30 years ago (Aschoff and Wever, 1962; Siffre, 19641. At that time, it was discov- ered that the intrinsic period of the human circadian pacemaker was typi- cally longer than 24 hours, implying that our internal clock required reset- ting to an earlier hour each day. Those studies further demonstrated that a number of behavioral rhythms, including the rest-activity cycle and circa- dian variations in cognitive and psychomotor performance, persisted in the absence of environmental time cues (Aschoff et al., 1971, 1972; Wever, 1979~. However, subjects in isolation from environmental time cues do not always choose to sleep and wake in synchrony with circadian variations in physiological variables, such as body temperature and cortisol secretion (Aschoff, 1965~. Whenever human subjects have been studied in environ- ments free of environmental time cues for more than two months, the timing of the self-selected rest-activity cycle loses synchrony with the stable, near- 24-hour oscillations that can be detected in physiological variables such as the body temperature cycle (Czeisler, 1978; Czeisler and Jewett, 19901. Yet even under such conditions of internal desynchrony, prominent circadian variations in alertness, cognitive performance, short-term memory, sleep tendency, spontaneous sleep duration, awakening and rapid eye movement (REM) sleep propensity all remain closely coupled to the body temperature cycle (Czeisler, 1978; Czeisler et al., 1980a, 1980b; Johnson et al., 1992; Zulley et al., 19811. Thus, no matter what sleep-wake schedule subjects choose to adopt in temporal isolation, they are, on average, most alert and perform best during their biologic day, as marked by the crest of the body temperature cycle; as might be expected, they are least likely to choose to sleep during those hours. Correspondingly, subjects are least alert, perform most poorly, and most often choose to be asleep during their biologic night,
SLEEP DISRUPTION AND FATIGUE 125 as marked by the temperature nadir. Moreover, these daily variations per- sist even when subjects are unaware of the time of day and are choosing to sleep and wake out of synchrony with these physiologic and behavioral rhythms. It is thus not surprising that field studies of human performance among shift workers demonstrate strikingly similar results. During the latter half of the night, responses to calls for service take the longest; performance on an F-104 flight simulator is worst; shooting range performance among mili- tary personnel is least efficient; mental arithmetic is slowest; alertness is lowest; short-term memory is markedly impaired; and the rate of single- vehicle truck accidents due to sleepiness is by far the greatest (Czeisler et al., 1986; Folkard et al., 1978; Froberg et al., 1975; Harris, 1977; Klein et al., 1977~. These circadian rhythms in performance variables parallel the wide variety of circadian rhythms in physiologic variables including daily variations in thermoregulation; hormone release; kidney, cardiac and respi- ratory function- which are controlled by the human circadian pacemaker (Babkoff et al., 1988; Czeisler, 1978, 1983; Froberg et al., 1975; Klein et al., 1977; Mills, 1974; Moore and Eichler, 1983; Wever, 1977~. Just as in all other eukaryotes, synchronization of the oscillations to the 24-hour day in humans is accomplished by means of exposure to the envi- ronmental light-dark cycle (Czeisler et al., 1989~. Thus, during the latter half of the night, humans are at their mental and physiological trough; maintaining high levels of alertness and performance at this time is exceed- ingly difficult. In fact, more than half of night shift workers report that they nod off or fall asleep at least once per week when working the night shift. Ironically, night shift workers who must stay awake during these unproductive hours then experience difficulty sleeping during their biologi- cal day. Sleep during daylight hours is disturbed, being shorter in duration and more frequently interrupted (Fores and Lantin, 1972~. These results from field studies are consistent with experimental data from subjects scheduled to attempt sleep at all times of day and night; sleep efficiency is at its lowest near the crest of the temperature cycle (Carskadon and Dement, 1975; Czeisler, 1978; Weitzman et al., 1974~. Thus, night shift workers typically suffer from misalignment of circadian phase. That is, they are attempting to stay awake and perform complex tasks at a time of day that the human circadian clock has scheduled the central nervous system to be asleep and then attempting to sleep at the scheduled time of peak arousal. In and of itself, this misalignment of circadian phase results in deteriora- tion of alertness and performance. In addition, because of the effect of circadian phase misalignment on sleep, night shift work inevitably results in acute and chronic sleep deprivation, which can independently impair performance.
26 WORKLOAD TRANSITION SLEEP DEPRIVATION Laboratory Studies The first demonstration of the deleterious effects of total extended sleep deprivation on human performance was reported nearly a century ago. At that time, 88-90-hour sleep deprivation experiments were carried out at the Iowa Psychological Laboratory by Patrick and Gilbert. They reported that grip strength, reaction time, memory, acuteness of vision, and calculation performance were adversely affected in some, but not all, subjects. Since that time, there have been a variety of studies investigating the deleterious effects of partial and prolonged sleep deprivation on the ability to perform sensory, physical, and cognitive tasks. As will be discussed in more detail in the following chapter, vigilance on both visual and auditory performance tasks has been found to be impaired after sleep deprivation (Hamilton et al., 1972; Horne et al., 1983; Sanders and Reitsma, 1982; Taub and Berger, 1973; Webb and Levy, 1984; Wilkinson et al., 1966~. Recall of verbal information is substantially reduced after even one night of sleep loss (Webb and Levy, 1984~. During the second night of sleep loss, subjects were able to complete only half their baseline average of problems solved on the Wilkinson Addition Test (Carskadon and Dement, 1979~. Loss of as little as 5 hours of nocturnal sleep results in a doubling of the number of errors in reading an electrocardiographic strip by resident physicians (Friedman et al., 19711. This decrease in performance appears to be due to impairments in perceptual encoding and storage, lapses of attention, and a decline in the ability to discriminate signals, rather than from a decline in willingness to respond (Home et al., 1983; Morris et al., 1960, Sanders and Reitsma, 1982; Torsvall and Akerstedt, 1987; Williams et al., 19591. Sleep tendency increases markedly with both partial and total sleep deprivation (Carskadon and Dement, 1979, 1981), such that ultimately sleep and/or attention lapses intrude involuntarily onto the waking brain (Williams et al., 1959~. Motor initiation and execution in response to visual stimuli are also impaired fol- lowing sleep deprivation, resulting in a failure to perceive information present in the functional visual field (Sanders and Reitsma, 1982~. Although addi- tional incentives (e.g., monetary rewards) can raise signal detection to nor- mal baseline levels for one night and the following day of sleep deprivation, by the second night detection rate falls regardless of the incentive offered (Home, 1988~. This may be why very long hours of work fail to increase total production in industrial settings (Chambers, 1961~. In addition to reduced ability to respond to sensory stimuli, both pri- mary mental performance and high-order cognitive functioning also show significant deficits following sleep deprivation (Hawkins et al., 1985~. In tests of mental arithmetic after sleep deprivation, speed of calculation is
SLEEP DISRUPTION AND FATIGUE 127 slower, calculation errors increase, and fewer calculations are attempted (Hamilton et al., 1972; Taub and Berger, 1973; Webb and Levy, 1984; Wilkinson et al., 19669. Furthermore, in memory tests subsequent recall of material learned under sleep deprivation is significantly less efficient than that of material learned under normal conditions (Folkard et al., 1977; Wilkinson, 1972~. Moderate sleep loss seems to cause deficits in memory trace forma- tion that are independent of the physiological lapses, or microsleeps, that will of course disrupt the initial perception of the material (Williams et al., 1966), and that occur with increasing frequency with each hour of addi- tional sleep deprivation. Finally, sleep deprivation impairs information pro- cessing, resulting in increased time requirements for making decisions (Asken and Raham, 19831. Several studies have employed protocols of extended wakefulness to investigate the interaction of the effects of sleep deprivation and circadian rhythmicity on reaction time, cognitive function, and shooting range perfor- mance (Babkoff et al., 1988; Carskadon and Dement, 1979; Dinges et al., 1987; Froberg et al., 1975~. These studies have found that performance in all variables declines during the first night of sleep deprivation, reaching a nadir just before dawn. During the second day of sleep deprivation, perfor- mance remains near this low level and may even improve somewhat, al- though it remains below normal baseline levels. However, during the sec- ond night of sleep deprivation, performance drops sharply, reaching levels much lower than those observed previously. Both sleep deprivation and adverse circadian phase contribute to substantial impairment of perceptual, motor, and cognitive functioning under such conditions. Operational Settings Many studies have documented the deleterious effects of both sleep loss and misalignment of circadian phase on performance and safety. Dur- ing the latter half of the night, there is an increased rate of errors in reading meters (Bjerner et al., 1955), longer delays in responding to calls by switch- board operators (Browne, 1949), and an increased rate of operational errors associated with falling asleep at the wheel by locomotive engineers (Kazutaka and Ohta, 1975~. These findings may help to explain the manyfold increase in the rate of single-vehicle truck accidents due to sleepiness that occur in the early morning hours between 4 and 6 am (Harris, 1977~. Yet in many operational settings, crews are willing to attempt to maintain sustained per- formance for even longer periods. These include hospital interns and resi- dents, whose performance is greatly impaired by such extended work hours (Asken and Raham, 1983; Friedman et al., 1971; Hawkins et al., 1985; Poulton et al., 1978), along with pilots, paramedics, and firefighters. They generally become impaired during the first night of sleep loss and are very
28 WORKLOAD TRANSITION ineffective after 40 to 72 hours without sleep. This is particularly true for people with command responsibilities (Belenky et al., 1987~. For example, Haslam (1985) found steady deterioration in the performance of soldiers who were allowed no sleep, 1.5 hours, or 3 hours of sleep each night for 9 consecutive days during a field exercise. Although physical fitness was not affected, vigilance and performance on detailed cognitive tasks deteriorated to 50 percent of premission levels. Although the group that did not sleep at all was unable to function after 3 days, the groups allowed 1.5 hours of sleep per night lasted for 6 days, and the group allowed 3 hours of sleep per night completed the 9-day study. Studies of military paratroopers' shooting performance over a 72-hour period showed a marked circadian variation in performance with a superimposed deterioration as sleep loss accumulated, consistent with the results of Babkoff et al. (1988) and Froberg et al. (1975~. Ainsworth and Bishop (1971) found that specific tank crew duties (e.g., those that required consistent, sustained alertness or perceptual-motor ac- tivities) were most sensitive to sleep loss in a 48-hour field test. Banderet et al. (1981) found performance decrements within the first 24-48 hours in a simulation of sustained operations in art artillery fire direction center. Sig- nificantly, they found that performance on self-initiated activities, such as planning and maintaining situational awareness, degraded most quickly. Even when operators are adequately rested, some types of missions are more fatiguing than others (e.g., day attack missions are more fatiguing for Army aviators than day medical evacuation (medevac) missions, while night scout-reconnaissance are the most fatiguing (Duncan et al., 1980~. In addi- tion, different factors contribute to the fatigue created by different missions. Exposure to hostile action was universally rated as the most important con- tributor. Additional factors were related to: (1) the mission (e.g., command pressure to complete the mission, duration of flying duty day, number of takeoffs and landings); (2) vehicle design (e.g., vibration, seating discom- fort); (3) scheduling (e.g., long or frequent standby periods, disrupted sleep schedules); (4) specific duties (e.g., tasks that are monotonous, impose high mental workload, or require monitoring heavy radio traffic); or (5) the envi- ronment (night versus day flights, weather problems). The Army has recog- nized these differences by specifying different recommended flight-time limitations depending on the type of mission flown (U.S. Department of the Army, 1985~. In computing flight time, one hour of daytime nap-of-the- earth flight was considered to be equal to 1.6 hours of daytime standard flight, and one hour of nighttime flight performed with night vision goggles was equivalent to 2.3 hours. These multipliers are used to determine the maximum recommended flight time and duty period in a 24-hour period (8 and 18 hours of day standard flight, respectively). The critical role of sleep deprivation in train crews was discussed in Chapter 2. Although data regarding impairment in performance from sleep
SLEEP DISRUPTION AND FATIGUE 129 deprivation in other team environments are less available, it is easy to generalize from the above results and envision that the sleep-deprived schedule that is characteristic of many medical personnel (particularly residents and interns tDoelp, 19893) may leave them especially vulnerable to medical errors (Gopher et al., 1989~. SLEEP INERTIA Sleep inertia occurs immediately on awakening and results in less ef- fective functioning than before sleep onset. This postsleep decrement in performance is seen in a variety of tasks, including simple and complex reaction time, grip strength, steadiness and coordination, visual-perceptual tasks, memory, time estimates, complex behavior simulation tasks, and cog- nitive tasks (Asken and Raham, 1983; Dinges et al., 1985; Downey and Bonnet, 1987; Feltin and Broughton, 1968; Seminara and Shavelson, 1969; Wilkinson and Stretton, 1971~. Sleep inertia is characterized by confusion and disorientation and usually results in high response latencies (Downey and Bonnet, 1987~. The effects of sleep inertia may take many minutes to dissipate and tend to be more intense after sleep deprivation and when wakenings occur at an adverse circadian phase (Dinges et al., 1985; Downey and Bonnet, 1987~. IMPACT ON PERFORMANCE IN EXTENDED-D[JTY OPERATIONS Army tank crews may be required to be in a state of readiness for up to 72 continuous hours prior to the onset of battle. Then, following an ex- tended period of inactivity, they may be required to abruptly begin a combat engagement. In such circumstances, the effects of circadian rhythmicity, sleep deprivation, and sleep inertia may conspire to markedly weaken the performance of the tank crew at this very critical time, threatening both their effectiveness and their safety. Performance reaches its daily nadir in the last hours of the night, just prior to dawn. Since physiologic sleep tendency is low during the evening, crew members may not attempt to sleep until they have already suffered from considerable sleep loss. The imposi- tion of strict work-rest schedules by the military will ensure that crew mem- bers' circadian pacemakers are mutually synchronized, resulting in all of them reaching their performance nadir simultaneously. Then, in the latter half of the night during extended periods of inactivity, crew members may find sleep irresistible. Struggling to stay awake, crew members may find themselves nodding off in much the same way as a very sleepy driver attempting to keep all four wheels on the road. Each time they do nod off, they will be further reducing their ability to respond to an immediate call to
130 WORKLOAD TRANSITION service, due to the lasting effects of sleep inertia. Added together, these factors can converge to create a critical zone of vulnerability from 3:00 am to 6:00 am, at just the times when night vision equipment would otherwise allow U.S. forces to enjoy a technical advantage over forces less well equipped. POTENTIAL COUNTERMEASURES There is no known technique available to sustain human performance at an acceptable level for 72 continuous hours. The effects of pharmaceutical agents meant to facilitate sleep often linger on after scheduled wake time, impairing performance, memory, or both. Similarly, pharmaceutical agents taken to promote wakefulness (e.g., caffeine, amphetamine) often interfere with the ability to catch some sleep when time permits and also directly impair psychomotor performance (Lipschutz et al., 1988~. As noted by Weiner (1985) and Pollard et al. (1990), amphetamines can aid individuals who are fatigued, but, after heavy use, sleep patterns may not return to normal for almost two months. According to Pollard et al., some laboratory studies have demonstrated a potential short-term effect; however, in the real world, use of amphetamines is likely to have serious negative effects. "As the effects of fatigue and lack of sleep increase, the operator may increase the dosage resulting in the build-up and finally pre- dominance of the negative side effects of the drug. These include blurred vision, dizziness, loss of coordination, paranoia, and irregular heartbeat fol- lowed ultimately by physical collapse" (Pollard et al., 1990:A-2~. Refer to Pollard et al. (1990) for a brief review of the effects of other central ner- vous system stimulants, central nervous system depressants, opiates, antide- pressants, and hallucinogens on performance. If such extended-duty performance is demanded, there will inevitably be a critical zone of vulnerability each day during which microsleep epi- sodes involuntarily intrude on the waking brain, leading to dangerous lapses of attention (Torsvall and Akerstedt, 1987~. In addition, if performance is abruptly required after a period of low workload, during which crew mem- bers are likely to have fallen asleep, then, because of sleep inertia, they may subsequently be disoriented, confused, and unable to consolidate memories or perform efficiently for at least 30 minutes after awakening. Such prob- lems may be exacerbated by the fact that the individual crew members often have little subjective awareness of the extent of their impairment under such circumstances. However, careful planning can result in the development of counter- measures that can reduce the impact of sleep disruption on the performance of tank crew members. Countermeasures that have been attempted in the face of such circumstances include caffeine (Lipschutz et al., 1988) and other stimulants, increased physical activity, naps, monetary incentives, diet,
SLEEP DISRUPTION AND FATIGUE 131 and intensive social contact. The duration of commercial airline flights has led the National Aeronautics and Space Administration to recommend scheduled cockpit naps as a potential countermeasure. Rosekind et al. (1991) exam- ined the effects of naps on long-duration transoceanic flights with three- person crews and found (1) most of the pilots were able to sleep during flight and had an average nap time of 26 minutes; (2) compared with a control group that was not given the opportunity to nap, the pilots who napped responded more rapidly to vigilance signals in the cockpit; and (3) the pilots who napped were not disrupted in their ability to sleep after the flight terminated by having gotten the extra nap sleep during flight. This measure has been found to be so effective that the Federal Aviation Admin- istration has recently modified regulations to allow scheduled napping in the cockpit. While these techniques can mitigate the deterioration of performance on the first night, none is effective in overcoming the impairments of per- formance that occur on the second or third nights of continuous operation. Recent research demonstrates that properly timed exposure to bright light and darkness can induce complete physiologic adaptation to night work and day sleep within 2-3 days (Czeisler et al., 1990~; however, even this new technology can only reschedule, but not eliminate, the daily trough in cog- nitive and psychomotor performance. Nonetheless, the use of properly timed exposure to bright light (both natural and artificial) and strict scheduling of exposure to darkness could enable commanders to disperse periods of vul- nerability either within individual tank crews or across tank divisions. In that way, there would be no one time at which an entire division would be most vulnerable to the effects of sleep loss and fatigue. Dinges and colleagues have demonstrated that the strategic placement of a single 2-hour nap can significantly reduce (but not eliminate) the peri- odic decrements of alertness and performance that occur during 54 hours of sleep deprivation (Dinges et al., 1987~. In order to reduce the consequences of all crew members suffering from the effects of sleep inertia simulta- neously, such preemptive naps should be scheduled in a staggered manner. Furthermore, naps must be a minimum of 10 minutes in duration in order to begin to restore the decrements due to sleep loss (Naitoh, 1980~. In terms of minimum sleep requirements, as noted above, Haslam (1985) found that soldiers allowed as little as 3 hours of sleep per night were able to continue functioning (although not at optimal levels) during a 9-day field study, whereas soldiers who did not sleep at all were judged to be militarily ineffective after three days. Krueger et al. (1985) found that Army aviators were able to maintain adequate flight control and navigation, but they began to make occasional judgment errors during 14 hours of precision instrument flight in a simulator on each of 5 successive days with only 4 hours of sleep each night.
32 WORKLOAD TRANSITION Leadership also plays a critical role in moderating the potential effects of sleep disruption. For example, Haslam (1985) found that the leadership of an experienced noncommissioned officer allowed his platoon to sustain their performance for a longer time than could other, equally sleep-deprived platoons. It is the responsibility of the team leader to establish a work-rest schedule that ensures that each team member gets adequate rest, if the situation allows. DeSwart (1989) describes sleep management as a critical element of stress management techniques adopted by the Royal Netherlands Army. In addition, the team leader must get adequate rest to be able to perform his own duties. During a mission, the team leader must monitor the status of each team member, relieving or reducing the responsibilities of team members who are too tired to perform their duties. Finally, it has been reported that individuals who sleep 9-10 hours per night for a week prior to sleep deprivation score consistently higher on performance tasks than those who have only slept 7-8 hours per night (Taub and Berger, 1973, 1976~. Therefore, strict enforcement of a 10-hour sleep regimen in total darkness during the weeks of rising tensions that often precede the deployment of armored personnel carriers could result in sub- stantial improvements in crew performance. Certainly, cumulative sleep dep- rivation should be avoided at all costs during such periods, since many studies have documented the impairments of performance associated with the habitual restriction of nocturnal sleep (Hamilton et al., 1972~. While none of these countermeasures can eliminate the effects of this powerful homeostatic regulatory mechanism on human performance, they can dissi- pate the impact of sleep loss and misalignment of circadian phase and mini- mize their deleterious consequences during the critical transition period from inactivity to combat. REFERENCES Ainsworth, L.L., and H.P. Bishop 1971 The Effects of a 48-hour Period of Sustained Field Activity on Tank Crew Perfor mance. Alexandria, Virginia: Human Resources Research Organization. Aschoff, J. 1965 Circadian rhythms in man: A self-sustained oscillator with an inherent frequency underlies human 24-hour periodicity. Science 148:1427-1432. Aschoff, J., M. Fatranska, H. Giedke, P. Doerr, D. Stamm, and H. Wisser 1971 Human circadian rhythms in continuous darkness: Entrainment by social cues. Science 171:213-215. Aschoff, J., H. Giedke, E. Poppet, and R. Wever 1972 The influence of sleep interruption and of sleep deprivation on circadian rhythms in human performance. In W.E. Colquhoun, ea., Aspects of Human Efficiency: Diurnal Rhythm and Loss of Sleep. London: English University Press. Aschoff, J., and R. Wever 1962 Spontanperiodik des menschen bie ausschuluss alter zeitgeber. Die Naturwissenschaften 49:337-342.
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