This is an excerpt from Advanced Environmental Exercise Physiology-2nd Edition by Stephen S. Cheung & Philip N. Ainslie.
Sleep deprivation is one of the most insidious health issues facing modern society. Technological advances and increased work demands, both in and out of the office, along with family and social responsibilities, often negatively impact sleep schedules. Attaining and maintaining sleep quality becomes problematic in occupational settings that require shift work. This problem is observed across a wide range of professions. Nonoptimal sleep is a universal experience, whether as a student pulling an all-nighter, as a medical resident or nurse on extended shifts, or as a new parent. The first two instances would be examples of sleep deprivation, featuring the complete absence of normal sleep at normal times, or abnormally long periods of wakefulness. In contrast, a new parent would experience sleep restriction, where the total duration of sleep time is shortened and combined with frequent interruptions by the demands of a newborn baby. Sleep deprivation and sleep restriction in combination can occur during extreme recreational endurance events and military operations.
Sleep deprivation causes an impairment in endurance capacity, though the precise mechanism of fatigue remains unclear. For example, following one night of sleep deprivation, time to exhaustion on a progressive cycle test was decreased when compared to performance after normal sleep (Temesi et al. 2013). Ratings of perceived exertion (RPE) were higher during the initial 40 min of cycling with sleep deprivation, although RPE was similar at failure, which would suggest that the exercise at each stage felt harder with sleep deprivation. Interestingly, neither peripheral nor cortical stimulation force, nor maximal voluntary contraction force, were decreased with sleep deprivation compared to rested. This maintenance of capacity, along with findings that acute sleep deprivation or restriction causes slower time trial performance (when effort is voluntary and self-paced), further indicates the complex psychophysiological regulation of exercise, as discussed in chapter 2.
Strength and anaerobic exercise do not seem to be overtly influenced by sleep deprivation besides the aforementioned decrease in motivation. Twenty-four hours of sleep deprivation did not impair peak isometric or isokinetic knee extension torque in eumenorrheic women (Bambaeichi et al. 2005). National-caliber weightlifting athletes were tested for sleepiness using a battery of psychological tests before and after a night of normal sleep or overnight sleep deprivation. Strength tests were performed and testosterone and cortisol were measured immediately before and after testing, and 1 h posttesting. Sleep deprivation decreased vigor and elevated confusion, sleepiness, and mood disturbance. After normal sleep, cortisol concentrations, a stress hormone, were attenuated immediately after the strength test, and even 1 h later. However, despite the impaired psychological profile of the sleep-deprived athletes, no differences were reported across sleep conditions in any of the strength measures. The authors concluded that for these elite athletes, a single bout of sleep deprivation did not negatively affect strength or exercise capacity. That said, the effect of sleep deprivation on sports heavily reliant on skill appears to be equivocal, and dependent on the task. For example, in a soccer-specific test battery, 24 h sleep deprivation decreased scores in continuous kicking—a test of frequency of accurate target kicks in a set time—but not in several other skills tests such as ball juggling and dribbling (Pallesen et al. 2017). No impairment was observed in 20 or 40 m sprint times, further suggesting that sleep deprivation may affect endurance capacity to a greater extent than anaerobic or power-dominant activities.
Sleep is critical in facilitating general recovery, and recovery from training or hard exercise. High-intensity cycling intervals in the evening cause increased feelings of tiredness and muscle soreness, decreased motivation to train, lower systolic blood pressure, and lower maximal heart rate even after a normal night of sleep (Rae et al. 2017). Restricting sleep to 3.8 h resulted in increased sleepiness and decreased motivation to train both 12 and 24 h afterwards, with decreased peak power output during subsequent interval training at 24 h. Interestingly, blood markers of muscle damage, immunoglobin secretion, and white blood cells levels did not suggest significant impairment in immune function or elevated muscle damage from acute sleep restriction. What cannot be determined from the current literature, however, are the effects of sustained sleep deprivation on long-term training and athlete development. In a survey on 24 athletes from a Bobsleigh Canada Skeleton team, 78% scored above the threshold for poor sleep quality on the Pittsburgh Sleep Quality Index (Samuels 2009). Poor sleep may be especially problematic in the long-term development of athletes during adolescence, with the additional physiological stress of puberty superimposed on training.
Military training is another field model for sleep deprivation research, especially the sustained special forces operations during which sleep is often reduced to minimal levels for days or weeks at a time. A simulated 72 h military operation featuring both prolonged sleep restriction to only 2 h daily and severe daily caloric deficit (–1,600 kcal) (Nindl et al. 2002) found that while squat jump power decreased, no decrements were observed in bench press power, and performance on operationally-relevant tasks such as marksmanship and grenade throwing were not impaired. In a field study over 61 days of U.S. Army Ranger training, one group of qualified military candidates endured sleep deprivation, receiving only 4 h of sleep daily, and a caloric deficit of −850 kcal (Young et al. 1998). Despite the prolonged stress, all subjects successfully completed the full training program, demonstrating that exercise capacity can be maintained even in extreme situations of protracted sleep and energy deficit for many fit, well-trained, and highly motivated individuals. While such military studies are excellent applications for sleep deprivation research, using them as experimental models is difficult because of the intertwined nature of extreme and continuous exercise, nutritional deficits, sleep deprivation, and psychological stress, and the difficulty in measuring changes to these variables in the field.
In a nonmilitary setting, Lucas and colleagues (2008) studied athletes participating in the Southern Traverse adventure race in New Zealand in 2003. This race features more than 400 km (250 mi) and 120 h of competition involving mountain biking, kayaking, orienteering, and coasteering. During the race, competitors demonstrated a significant drop-off in self-paced exercise intensity, from an average of 64% maximal heart rate (HRmax) in the first 12 h of the race to an average of 41% HRmax from 24 h through the rest of the race. This was true for both male and female members, in the winning and the last-place teams, suggesting that the downregulation in exercise intensity was not a by-product of race placement. It can be argued that the decrease was due to general pacing errors; an overly high initial pacing is common to most time trials of any distance. However, heart rate responses did not differ during laboratory exercise tests performed before and after the competition, suggesting there was no autonomic impairment in cardiovascular regulation per se. Notably, subjective ratings of exercise intensity were significantly higher post-competition, likely due to changes in psychological motivation following such a strenuous event. Core temperature during competition remained within normal values despite wide-ranging environmental conditions. Immediately after the competition, upper respiratory problems and skin wounds, along with gastrointestinal problems, were more prevalent than before the competition. Mood changes were also common. Unfortunately, it is a feature of these field-based events that it is difficult to isolate various physical effects from the effects of sleep deprivation alone, and nearly impossible to generalize how sleep deprivation affects performance in sustained exercise without objective sleep measures.
In summary, it appears that exercise capacities are only somewhat impaired, and most performance decrements arise from subjective impairments. Ultra-endurance exercise and sustained operations with significantly reduced sleep time and quality can be tolerated with minimal decrements in exercise and operational capacity, at least in highly motivated and trained participants. Levels of motivation and psychological characteristics seem to play a role in determining the effect of sleep deprivation on performance impairments in such environments, so extrapolating these results to less motivated groups may not be valid. Relatively little work has focused on the direct effect of fitness on sensitivity or response to sleep deprivation.
Influence of Ramadan on the Sleep–Wake Cycle
Like many religions and cultures that practice voluntary fasting, Muslims all over the world fast every year during the holy month of Ramadan that lasts between 29 and 30 days. Fasting during Ramadan is the fourth pillar of Islam for about 1.7 billion Muslims worldwide. During Ramadan fasting, Muslims are required to refrain from food or fluid intake between sunrise and sunset. This practice displaces energy intake and hydration to the hours of darkness and partially reverses the normal circadian pattern of eating and drinking. As illustrated in figure 14.4, during Ramadan fasting, factors such as sleep, daytime sleepiness, and circadian rhythms are all influenced (Qasrawi et al. 2017). For Muslim athletes, in part due to restrictions on food and fluid consumption during daylight and to reductions in the sleep–wake cycle, the continuation of training and competition during Ramadan can sometimes lead to compromises in physical and cognitive performance (Shephard 2012).
As mentioned previously, circadian rhythms play an important role in occupational health and safety. Shift work involves an alteration of the typical circadian rhythm. The circadian period can be phase-advanced (by moving from a normal daytime schedule to a nighttime schedule) or phase-delayed (by moving from a daytime to an evening schedule). Working in shifts that differ from the traditional daylight working hours is common in occupations such as health care, emergency response, utilities, and transport. Each of these occupations also involve the potential for accidents from sleep deprivation due to physical fatigue or cognitive impairment.
Decreased arousal and cognitive capacity are hallmarks of sleep impairment common to shift work. These impairments extend from simple cognitive domains, like response time, to complex executive functions. Mood, risk-taking behavior, and working memory are all worsened by sleep deprivation. In professional settings, this can lead to accident or injury. Research indicates that naval officers demonstrate reduced moral reasoning and decreased ability to anticipate tactical issues when sleep deprived. Similarly, in medicine, where 24 to 36 h shifts are routine, epidemiological research highlights the increased risk for medical errors as the workday progresses and the health professionals work additional shifts.
Unlike transmeridian travel, shift work creates a permanent mismatch in the sleep–wake cycle between endogenous rhythms, environmental cues, and social constraints. This makes it difficult if not impossible to fully acclimatize to shift work at night. Initial exposure to shift work elicits many of the cardiovascular and neuroendocrine changes observed in clinical conditions such as chronic fatigue syndrome. Circadian impairment is further exacerbated by the regular rotation of working shift times in many of these occupations, with the occurrence of only a few shifts before another work schedule is imposed and re-adaptation is again required. For example, circadian rhythms can only shift by one or two hours per day when a person changes his or her sleep schedule, so resynchronization (entrainment) takes several days to occur. Therefore, it is unsurprising that shift workers have a high prevalence of insomnia and other sleep disorders. Symptoms of nonadaptation to shift work include decreased cognitive performance across a range of tasks, along with decreased attention and vigor. Such cognitive impairments appear to progressively worsen with chronic exposure to shift work, although, thankfully, removal from a shift work regimen seems to provide some degree of recovery. Experience on the job does not seem to confer protection against cognitive impairment, but rather the negative physiological effects of shift work are cumulative.
Other hidden societal costs of shift work include an increased risk of depression, addictions, and mental health issues. For example, nursing students exposed to shift work for the first time exhibited marked increases in psychological indices typical of depression, such as disturbed sleep and appetite, along with lethargy and poor concentration (Healy et al. 1993). These findings were above and beyond the slight elevation in baseline values recorded in controls due to the stress of nursing training itself. Furthermore, exposure to shift work induced subjective feelings of helplessness, eroded perceptions of social support, and increased feelings of criticism as well as psychosomatic complaints. The requirements of shift work during weekends, when family and social expectations are often greater, may prove especially problematic.
It appears that physical responses to shift work are a matter of coping rather than truly adapting. Temporary strategies should accommodate what is required to minimize the effects of this shift in cycles. The presence of shift work is a strong predictor of work–home conflict, with more conflict occurring when shift work takes place on weekends. A rotating shift schedule was highly predictive of unfavorable job attitudes and poor job satisfaction ratings. Individual situations and tolerance to shift work and rotation can be highly variable, so it is useful to develop methods of assessing one’s response and tolerance to variable schedules before customizing individual schedules where feasible. For example, cortisol concentration, which also exhibits a circadian rhythm, has been proposed as a monitoring tool for assessing adaptability to shift work and also to customize shift work to the individual.
Research Focus: Sleep Deprivation and Thermoregulation
Given the strong relationship of decreased body temperature with overnight sleep, one interesting avenue of research is whether heat stress during the day affects sleep quality. In a multiday wildland firefighting simulation, high ambient temperatures during daily activities, coupled with a slightly elevated sleeping temperature and sleep restriction, did not negatively affect sleep architecture and quality beyond that resulting from sleep restriction and temperate daily and nighttime temperatures (Cvirn et al. 2017).
Alternatively, can sleep impairment affect the circadian pattern of core temperature and thermoregulation itself? The response to cold stress following sleep deprivation appears to be relatively minimal. Early research suggested there was a decrease in resting core temperature following 50 h of sleep deprivation, but that core temperature was rapidly restored with the onset of exercise (Kolka et al. 1984). Importantly, self-selected exercise intensity to achieve thermal comfort did not differ across sleep conditions in this study, suggesting that perception of thermal state and fatigue was not altered. This lack of physiological impairment to the cold response was supported by more recent evidence that 53 h of continuous sleep deprivation did not impair either physiological response to 4 h of 0 °C air exposure (Oliver et al. 2015) or the rate of rewarming from cold exposure in 10 °C air (Esmat et al. 2012). Finally, in the Arctic survival simulation study discussed in chapter 5 (Haman et al. 2016), participants were able to maintain adequate shivering overnight despite sleep deprivation and caloric restriction, such that core temperature retained a typical circadian pattern despite being forced down to a lower value by the cold environment.
While sleep deprivation has been proposed to contribute to heat-related illnesses, its influence on physiological responses to heat stress remains equivocal. Early research reported that males had a higher core temperature during exercise in heat stress following sleep deprivation, likely due to reduced sweating. More recent work studying females have reported no change in core temperature responses to a heat stress test following sleep deprivation. However, perceived symptoms of heat illness were consistently higher with sleep deprivation, suggesting an altered perception of heat stress.