Li Ling Lim
Department of Neurology, Sleep Disorders Unit, Singapore General Hospital, National Neuroscience Institute, Singapore, Republic of Singapore
Department of Neurology, Sleep Disorders Center, Cleveland Clinic Foundation, Cleveland, Ohio, U.S.A.
With the invention of the light bulb and ready availability of electrical power, sleep loss has become increasingly prevalent in industrialized nations. Air travel across multiple time zones, globalization of commercial markets, shift work, 24/7 services, television, and the Internet have contributed to longer waking hours and reduced sleep. The mean number of hours of nocturnal sleep has fallen steadily in the last century from about 9 to 6.9 hours in 2005, based on the most recent National Sleep Foundation Poll (1). The proportion of Americans sleeping 8 or more hours on weekday nights has fallen steadily from 38% in 2001 to 26% in 2005. Therefore, sleep deprivation (SD) has become one of the most important yet understated public health issues of modern times.
Restless legs syndrome (RLS) can result in marked SD. In one tertiary center that specializes in RLS, the SD seen in RLS on polysomnography was greater than that of any illness except mania and fatal familial insomnia (Allen, personal communication). In this chapter, we will discuss the medical and neuropsychiatric consequences of SD.
How Much Sleep Do We Need?
Sleep needs vary from one individual to another and with age. Most adults require 6 to 10 hours of sleep per night to function optimally. Experts often recommend at least 8 hours of sleep, estimating that 1 hour of sleep is needed for every 2 hours awake (2). A recent dose-response sleep restriction experiment showed that cumulative wakefulness in excess of 16 hours resulted in performance lapses, whereas cognitive performance remained stable with 8 hours of sleep (3). With the exception of ''short sleepers'' who function well with less sleep than expected for their age, most people sleeping fewer than 5 to 6 hours per night are not getting enough sleep (4). In the 2005 National Sleep Foundation Poll, adult Americans reported that a minimum of 6.5 hours of sleep on average was needed for optimal daytime functioning. The median sleep duration was 7 to 7.9 hours and 16% of respondents slept less than 6 hours on weekday nights.
Large epidemiological surveys have consistently demonstrated a U-shaped association between sleep duration and mortality, with a nadir at 7 hours (5,6). Life expectancy is significantly reduced in people sleeping less than 4.5 hours per night and the mortality risk is even greater in subjects sleeping over 7.5 hours per night, even after controlling for comorbidities. These findings strongly suggest that sleep needs should be individualized and that a standard recommendation of 8 hours of sleep per night should not be uniformly made.
While Dement has stated ''all wakefulness is sleep deprivation,'' SD usually refers to the ''failure to obtain sufficient nocturnal sleep to support normal alert wakefulness'' (2). Volitional chronic partial SD with its detrimental effects, classified as behaviorally induced insufficient sleep syndrome by the International Classification of Sleep Disorders 2, is the most common form of SD in humans (7).
Sleep loss can be voluntary or produced by a variety of environmental factors, or medical, psychiatric, and sleep disorders which disrupt sleep, including chronic pain, depression, sleep apnea, and RLS. High-frequency sleep fragmentation due to repeated arousals from sleep produces nonrestorative sleep and results in similar neurobehavioral consequences and performance deficits as voluntary SD (8). Sleep fragmentation and SD both result in daytime sleepiness, decreased psychomotor performance, and comparable physiological changes (9). In combination, voluntary sleep restriction and sleep fragmentation due to underlying medical or sleep disorders, both individually common occurrences, are likely to have synergistic and therefore even greater negative impact.
Volitional, but unintentional lack of sleep, is often chronic and a result of the excessive social and occupational demands of modern life and inadequate sleep hygiene. Excessive daytime sleepiness is the chief complaint, associated with a host of secondary symptoms such as fatigue, low energy, poor concentration, inattention, irritability, and dysphoria. The nocturnal habitual sleep period is shorter in duration than expected for age, leading to the common habit of sleeping in on weekends to ''catch up.'' Polysomnography features include a short sleep onset latency, high sleep efficiency, and prolonged total sleep time if allowed to sleep in. A short sleep latency with or without multiple sleep onset rapid eye movement (REM) sleep periods (SOREMPs) is typically observed on multiple sleep latency testing (MSLT).
SD can be acute or chronic, partial or total. While prolonged total sleep loss is rarely encountered in humans outside of a research setting, chronic partial sleep loss is a frequent occurrence, affecting an estimated one-third or more of normal adults, with especially severe sleep loss seen in night-shift workers (10).
Although the optimal duration and function of sleep remain unclear, the detrimental effects of SD have been studied for over a century. Human SD studies are discussed in detail below. Animal studies are also briefly reviewed for comparison. Experiments have addressed the effects of total, partial, and selective SD and recovery sleep.
The effects of total SD and partial SD in humans are qualitatively similar, with differences mainly in the time course of symptom evolution (3). Selective deprivation of specific stages, such as REM or slow wave sleep (SWS), has not consistently been shown to cause major psychological disturbances or performance decrements (11).
Total SD experiments, including one of the first in 1896 (90 hours) and the longest documented (264 hours or 11 days), have consistently produced the following effects (12,13):
1. Sleepiness, especially severe at night (circadian variation), which can be overcome with arousal (e.g., external stimulation and physical activity) and brief sleep episodes (''microsleep''), which are usually imperceptible to the subject.
3. Impaired performance in tasks requiring close attention such as vigilance testing, but not visually stimulating motor activity such as exciting arcade games. Performance can be transiently improved with motivation and incentives.
4. Neurological disturbances including visual illusions and hallucinations, blurred vision, ptosis, disorientation incoordination, slurred speech, memory lapses, delusions, slowed mentation, and difficulty in thinking.
5. Mild decrease in body temperature, but no major physical or metabolic abnormalities.
6. Relatively short recovery sleep over 1 to 3 nights with apparent return to baseline alertness and function, comprising approximately 25% of total sleep lost: two-thirds of lost SWS, one-third of lost REM sleep, and less than 10% of lost light non-REM (NREM) sleep.
A recent large-scale, dose-response chronic sleep restriction experiment compared the neurobehavioral and physiological effects of partial and total SD (3). Chronic partial SD of 4 to 6 hours per night over 14 days resulted in significant cumulative, dose-dependent deficits in cognitive performance and an increase in self-reported daytime sleepiness. The cognitive effects of chronic partial SD of 6 hours or less per night was equivalent to that produced by 2 nights of total SD. The rate of deterioration was inversely related to the duration of sleep time, with total SD producing more rapid decrements. Relatively moderate sleep restriction seriously impaired waking neurobehavioral functions in healthy adults. The subjects were largely unaware of these.
Effect of Sleep Deprivation on Alertness, Mood, and Performance
Sleepiness is the most evident consequence of SD. Carskadon and Dement performed the earliest studies using the MSLT to measure the effects of total SD on young adults. A marked and persistent decline in mean sleep latency (MSL) approaching zero and returning to baseline after the second recovery night was observed. Performance and mood similarly declined following two nights of total SD (14). The same authors reported similar relative reductions in MSL with partial SD, when the normal sleep period was shortened by only 30 to 60 minutes (15). When partial SD was prolonged, the MSL was further reduced.
These early observations of the cumulative effect of partial sleep restriction on waking functions have since been confirmed in other studies and have led to the concept that SD results in the accumulation of a ''sleep debt'' that must be repaid in order to restore normal alertness (16).
A meta-analysis of 19 studies reported over the last several decades concluded that mood is more vulnerable to SD than either cognitive or motor performance (17).
Mood ratings of sleep-deprived subjects were found to be more than three standard deviations worse than non-sleep-deprived individuals.
One study of partial SD for seven consecutive nights in healthy young adults reported increased mood disturbances and negative mood states, which the authors postulated may be exacerbated by the stresses of coping with daytime sleepiness, impaired cognition, and the compensatory effort required to stay awake and motivated during the day (16). In physician trainees who slept an average of only 5.95 hours daily, a higher incidence of depression was found in the internship year than expected for age group, with risk factors being female sex, unmarried status, and a past history of major depression (18). In another study, surgical residents whose sleep was acutely restricted to less than 4 hours per night were found to be more depressed, tense, and angry than in the rested state (19).
Interestingly, in depressed subjects, SD has the reverse effect (i.e., antidepres-sant), which is short-lived and reversible in most cases after a single night of recovery sleep (20). Although the mechanism remains unclear, partial, total, and selective SD have all been used successfully to treat depression (21).
Sleep loss has consistently been shown to significantly impair psychomotor performance. One meta-analysis found that the mean level of functioning of sleep-deprived subjects was comparable to that of only the ninth percentile of non-sleep-deprived subjects (17). Cognitive performance was found to be more affected than motor performance, and mood was much more affected than either cognitive or motor performance.
Performance deficits involving vigilance are evident after only two nights of mild sleep restriction to 5 hours (15,22). With longer duration (60 days) of comparable degrees of sleep loss, progressive decline in vigilance is seen (23). Similar cumulative deficits in alertness and performance have been shown in partial and total SD, with greater and more rapidly accumulating deficits with total SD.
In contrast to vigilance tasks, a controlled study of 11 male subjects showed that up to 60 hours of sleep loss did not significantly impair physical performance testing, including isometric and isokinetic muscular strength and endurance, and cardiovascular and respiratory responses to treadmill running (24). Military studies have also shown that physical performance remains relatively unaffected in spite of severe sleep restriction (0-3 hours of sleep) for up to nine days (25,26).
The impact of SD on performance is particularly relevant in everyday tasks such as driving and in certain occupations where chronic sleep restriction is common. The U.S. Department of Transportation recently identified fatigue as the number one safety problem in transportation operations, costing billions of dollars annually (27). In June 2003, the state of New Jersey was the first in the United States to pass legislation prohibiting driving a motor vehicle while impaired by lack of sleep (28).
Impairment in simulated driving tests after modest sleep loss has been shown to be comparable to that produced by alcohol. In one study, 18.5 and 21 hours of wakefulness produced deficits of the same magnitude as 0.05% and 0.08% blood alcohol concentration, respectively (29). The combined effect of sleep restriction and alcohol is additive, reducing alertness, and impairing simulated automobile driving in healthy young men, even with low alcohol intake and safe alcohol concentrations below 0.08% blood alcohol concentration, the legal intoxication level in most states (30,31). The effects of regular sleep loss on driving are further exacerbated by coexisting disorders like sleep apnea, the treatment of which with positive airway pressure therapy has been shown to reduce the risk of traffic accidents (32).
Certain high-risk occupations such as physician trainees are particularly vulnerable to chronic partial SD. One study showed that interns slept an average of less than 6 hours per night (18). Acutely sleep-deprived house officers exhibited significant deficits in mental tasks involving rote memory, language, numeric skills, and high-level cognitive tasks (33). A study of rested vs. fatigued anesthesia residents showed significant deterioration in tasks requiring vigilance in a simulated monitoring situation (34). Physician trainees were also shown to have a higher incidence of drowsy driving and sleep-related motor vehicle accidents, especially post-call, than a control group of rested physicians (35).
Factors Affecting the Impact of SD on Alertness, Mood, and Performance
The impact of SD on alertness, mood, and performance is influenced by extrinsic factors such as those causing arousal, and intrinsic factors such as sleep homeo-stasis, circadian rhythm effects, and age.
Because our propensity to sleep is a balance of sleep debt and level of arousal, it stands to reason that extrinsic factors that increase arousal can counteract and even mask the effects of sleep loss. Physical activity or exercise and stimulant drugs, including caffeine, are common examples.
In a study of 12 normal adults, a five-minute walk produced a significant elevation of heart rate, a measure of physiological arousal, and prolongation of mean sleep onset latency on the MSLT (36). The performance benefits of exercise are short-lived. Longer periods of physical activity do not improve overall performance in sleep-deprived individuals (37). Bright light, noise, and ambient temperature are other extrinsic factors that have also been shown to increase arousal (38-42). For example, a driving simulator study using noise (radio) and cold air as a countermeasure to sleepiness in partially sleep-deprived young adults demonstrated improvement in subjective sleepiness scores and a trend toward reduction of ''lane-drifting'' incidents. The increased alertness was, however only marginal and transient, and comparatively less effective than a short nap (<15 minutes) or 150 mg of caffeine (42).
There is an extensive body of literature demonstrating the effects of stimulant drugs on mood, alertness, and performance following SD. Amphetamines reverse the effects of 48 hours SD on sleep latency and behavior in a dose-dependent manner (43). Methylphenidate increased MSL, improved performance on vigilance tasks, as well as depression and fatigue scores in a placebo-controlled study (44). Similar alerting and performance benefits have been demonstrated with modafinil (45,46). Caffeine has also been shown to reverse the effects of SD on alertness and mood. A double-blind study of normal subjects sleep deprived for 49 hours showed that a 600 mg dose of caffeine significantly increased sleep latency and decreased sleepiness, fatigue, and confusion to near resting levels (47).
Performance can also be affected by factors related to the test situation such as level of interest in a given task and by incentives such as financial rewards for good performance (48,49). Immediate feedback produces momentary arousal that can improve performance, while increasing task duration has the opposite effect (50). Increasing the difficulty and complexity of a task also adversely affects performance (51,52). Generally, the most profound deficits are observed on tasks that are long in duration, newly learned, difficult, externally paced, devoid of feedback, and have a memory component (51,53-55).
Sleep Homeostasis, Circadian Rhythm, and Other Intrinsic Factors Wakefulness accumulates sleep debt, which leads to increased sleepiness that, in turn, is discharged during sleep. This homeostatic process is modulated by the circadian pacemaker, which regulates level of alertness, usually lowest during the nocturnal hours and in the mid-afternoon (56) (Chapter 3).
A study of the effects of 72 hours of SD in 12 subjects, in whom levels of alertness, affect, motivation, and cognition were measured, showed that most of these variables were significantly affected by the number of days of sleep loss and all were significantly affected by the hour of the day (57). The peak time for self-reported complaints was in the early morning between 4:00 a.m. and 8:00 a.m., while the fewest symptoms were reported in the late afternoon between 4:00 p.m. and 8:00 p.m.
Sleep homeostasis is affected by daytime napping, which reduces sleep debt. Taking a nap before a period of prolonged wakefulness transiently improves alertness, mood, and performance in a dose-related manner (58). The beneficial effects of three- to four-hour prophylactic naps are comparable to a moderate dose of caffeine (300 mg), and the combined effect of naps and caffeine is additive (59).
Intrinsic factors such as age may have an impact, although relatively minor, on the effects of sleep loss. Tests of performance and alertness in normal older sleep-deprived subjects reveal loss of performance and alertness similar to that seen in younger individuals. However, older individuals may tolerate sleep loss with less decrease in ability compared to baseline than young adults, and may recover function faster than young adults (60).
Neurologic Effects of Sleep Deprivation prolonged SD produces reversible neurologic signs such as slurred speech, hand tremor, ptosis, nystagmus and abnormal corneal, gag, and deep tendon reflexes (61,62). Sleep loss also affects immediate recall and short-term memory, causing memory deficits on testing similar to that seen with alcohol intoxication and aging, postulated to be due to impaired encoding and retrieval of information (55,63). On positron emission tomography studies, 24-hour SD resulted in a global decline in brain activity, most marked in the thalamus and prefrontal and posterior parietal cortices, areas mediating attention, and higher-order cognitive processes (64).
Electroencephalographic (EEG) studies have shown a progressive slowing of the waking alpha rhythm in proportion to the duration of sleep loss, with loss of alpha reactivity after prolonged SD (65). In one study, a correlation between theta and delta activity during wakefulness and impaired performance following SD was observed (66). Microsleep episodes lasting up to 10 seconds are also seen in the EEGs of healthy human subjects sleep deprived for 60 hours (67). Such microsleep episodes are ameliorated with stimulating agents such as modafinil (46).
SD has long been recognized as a precipitating factor for seizure recurrence in people with epilepsy. Patients with epilepsy frequently report SD as a seizure precipitant (68). SD is also often used as an activating procedure during routine EEG and long-term video EEG monitoring (69-71). Early studies in nonepileptic, mostly military populations who were profoundly sleep deprived, found a significantly higher incidence of seizures than in demographically similar populations who were not sleep deprived (72). EEG studies of both normal healthy subjects (65,73) and subjects with suspected epilepsy (74-77) have also demonstrated a clear increase in interictal epileptiform abnormalities following SD.
Despite the vast body of evidence supporting the multitude of adverse consequences of SD on neurologic function, similarly marked changes in other organ systems have not been found (78). Accelerated metabolism (and premature death) following partial and total SD seen in animals has not been demonstrated in humans. However, sleep-deprived humans do exhibit similar declines in body temperature and increased sympathetic tone (12,79). The range of physiological changes associated with SD in cardiovascular, respiratory, endocrine, and immune function is discussed.
Several epidemiologic studies have found an association between cardiovascular morbidity and chronic sleep restriction (6,80). In the Nurses' Health Study, women sleeping less than 7 hours per night had an increased risk of coronary events compared to those averaging 8 hours. Precisely, how chronic SD affects cardiovascular morbidity remains unknown.
Respiratory testing in healthy adults has shown small but significant decreases in forced vital capacity, hypoxic and hypercapneic ventilatory responses after SD (81,82). SD has also been shown to cause reversible worsening of respiratory events in normal infants and adults with obstructive sleep apnea syndrome (83,84).
With the exception of thyroid hormones, appreciable hormonal alterations have not been observed after total SD, including catecholamine, cortisol, or sex hormone levels (85). With partial SD, impaired glucose tolerance, elevated evening cortisol levels, and thyroid function abnormalities have been reported (79). Sleep loss can alter the pattern of secretion of hormones under circadian influence. For example, selective deprivation of SWS can diminish the secretion of growth hormone (86).
Human studies have demonstrated a variety of changes in immune function following SD. However, firm evidence for increased susceptibility to infections is lacking. Some reported findings include
1. enhanced ability of lymphocytes to produce interferon (87),
2. impaired natural killer cell activity and T-cell cytokine production (88),
3. decreased DNA synthesis of blood lymphocytes (89),
4. altered antigen uptake (90), and
5. decreased antibody responses in humans to viral vaccines (91,92).
Although the studies examining the effect of SD on immune function in humans are difficult to compare due to differences in methodology, it is generally believed that sleep loss does influence the immune system. Short-term SD may enhance immune responses, while chronic sleep loss is probably detrimental.
The earliest REM SD experiments were conducted in an attempt to determine the function and necessity of REM sleep. Initial interest in this area of research was stimulated by the Freudian belief that subconscious impulses and desires, if unexpressed as dreams during REM sleep, would cause a buildup of ''psychic pressure,'' and eventually lead to neurosis and psychosis. REM SD in rats induces hyperactivity, irritability, aggressiveness, stereotypy, and hypersexuality, which some authors consider a model for mania (93). In selective deprivation experiments in which human volunteers would be awoken at the first sign of REM sleep, REM sleep onset latency decreased and REM sleep duration increased progressively, eventually producing SOREMPs. REM SD caused confusion, bad temperedness, and severe sleepiness, but not psychosis (2). While animal studies have shown an association between REM sleep and memory and learning, firm evidence for this in humans is lacking. REM density has been reported to increase after training in humans, implicating REM sleep in memory processing (94). Yet, antidepressant drugs that suppress REM sleep are used commonly without apparent adverse effect on memory, suggesting that the role of REM sleep in human memory and learning is complex and poorly understood.
In comparison to selective REM deprivation, there are fewer studies addressing selective deprivation of SWS in humans. Selective SWS deprivation studies are difficult to perform and often affected by methodological problems. Selective SWS disruption, without reducing total sleep or sleep efficiency, has been associated with fatigue, decreased pain threshold, and reduced vigor (95). However, daytime performance testing following several nights of either REM or SWS deprivation has not been shown to result in apparent decrements (11).
The effect of SD on sleep architecture was studied in healthy subjects who underwent partial sleep restriction (50% or 4 hours) for four nights followed by three recovery nights of 8 hours (96). During the period of SD, there was decreased light NREM sleep (stages 1, 2) and REM sleep, but SWS was mostly unaffected. On the first recovery night, there was shortening of the sleep onset latency and increased total sleep time, including enhanced SWS and REM sleep. On the second recovery night, SWS duration approached normal while REM sleep rebound continued. Sleep architecture subsequently normalized by the third recovery night.
The effects of SD on alertness and performance reverse after 1 to 3 nights of recovery sleep, regardless of the duration of prior wakefulness (63,97,98). Studies of prolonged total SD have shown that the amount of recovery sleep required to restore baseline performance is only about 25% of the amount of sleep loss incurred (12,13). It is unclear whether this means that accumulated sleep debt need not be repaid in full as originally thought, or if residual deficits may be too small to be measurable or difficult to measure in typical SD paradigms.
Animal models of SD, employing in some cases long periods of sleep loss, have demonstrated more pronounced abnormalities than observed in sleep-deprived humans (99,100). Rats subjected to SD experience weight loss and assume a malnourished appearance in spite of increased food intake. Skin lesions (ulcers and discolored fur), increased energy expenditure due to increased heat loss, decreased body temperature during the late stages of deprivation, biochemical changes including increased plasma norepinephrine and decreased plasma thyroxine are observed. Eventual death ensues after an average of 19 days of near-total SD. Animals subjected to selective REM or SWS deprivation survive longer. The mechanisms leading to death after total SD are unclear and confounded by the effect of stress, which accompanies profound sleep loss. Proposed mechanisms include hypothermia, hypermetabolism, and impaired immunity, but none have been firmly proven. For example, septicemia involving opportunistic microbes, implicating impaired immunity in the hypercatabolic, malnourished, and ultimately life-threatening state associated with prolonged SD was described (101). However, subsequent studies demonstrated that elimination of the bacterial invasion did not prevent hypothermia or early death in rats subjected to total SD (102).
Studies of recovery sleep in surviving rats have shown near-complete reversal of SD-related changes in appearance, metabolism, temperature, biochemistry, and high energy expenditure (103). Recovery sleep is characterized by immediate and prominent REM sleep rebound, which contrasts with the SWS rebound seen in humans.
Human studies have consistently demonstrated significant detrimental effects of SD on alertness, mood, performance, and neurological function. Chronic SD has been associated with increased all-cause and cardiovascular mortality. Physiological effects are relatively minor and include changes in thermoregulation, respiratory, endocrine, and immune function. Chronic partial SD is often unrecognized and likely represents one of the most important public health issues in modern society. Dement considered ''the pervasive lack of awareness about SD a national emergency'' (2). The additive effects of alcohol, drugs, social and occupational demands, and the coexistence of common disorders that disrupt sleep, including chronic pain, depression, sleep apnea, and RLS, exacerbate the problem. The impact of SD on transportation and occupational safety, comparable to the effects of alcohol intoxication, are potentially catastrophic. Measures to address these concerns have included legislation against drowsy driving, later school start times, and restriction of duty hours for physician trainees. These measures underscore the serious health and socioeconomic consequences of sleep loss, but represent just a few of the initiatives required to manage this enormous problem.
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