Chronic Sleep Deprivation Siobhan Banks and David F
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Chapter Chronic Sleep Deprivation Siobhan Banks and David F. Dinges 6 Abstract (3) do not result in profound subjective sleepiness or full self- awareness of the cumulative deficits from sleep restriction. Chronic sleep deprivation, also referred to as chronic sleep The mechanisms underlying the sleep dose–response cumula- restriction, is common, with a wide range of causes including tive neurobehavioral and physiologic alterations during shift work and other occupational and economic demands, chronic sleep restriction remain unknown. Individual variabil- medical conditions and sleep disorders, and social and domes- ity in neurobehavioral responses to sleep restriction appear tic responsibilities. Sleep dose–response experiments have to be as large as those in response to total sleep deprivation found that chronic sleep restriction to less than 7 hours per and as stable over time, suggesting a traitlike (possibly night resulted in cognitive deficits that (1) accumulate (i.e., genetic) differential vulnerability to the effects of chronic sleep become progressively worse over time as sleep restriction restriction or differences in the nature of compensatory brain persists), (2) are sleep-dose sensitive (i.e., the less sleep that responses to the growing sleep loss. is obtained, the faster the rate at which deficits develop), and Chronic sleep restriction occurs frequently and results by problems in experimental design.8 Since 1997, experi- from a number of factors, including medical conditions ments that have corrected for these methodological weak- (e.g., pain), sleep disorders, work demands (including nesses have found markedly different results from those extended work hours and shift work), and social and earlier studies, and have documented cumulative objective domestic responsibilities. Adverse effects on neurobehav- changes in neurobehavioral outcomes as sleep restriction ioral functioning accumulate as the magnitude of sleep loss progressed.10 This chapter reviews the cognitive and neu- escalates, and the result is an increased risk of on-the-job robehavioral consequences of chronic sleep restriction in errors, injuries, traffic accidents, personal conflicts, health healthy individuals. complaints, and drug use. Chronic sleep restriction, or partial sleep deprivation, INCIDENCE OF CHRONIC SLEEP has been thought to occur when one fails to obtain a usual amount of sleep.1,2 Half a century ago, Kleitman first used RESTRICTION the phrase sleep debt to describe the circumstances of delay- Human sleep need, or, more precisely, the duration of ing sleep onset time while holding sleep termination time sleep needed to prevent daytime sleepiness, elevated sleep constant.3 He described the increased sleepiness and propensity, and cognitive deficits, has been a long-standing decreased alertness in individuals on such a sleep–wake controversy central to whether chronic sleep restriction pattern, and proposed that those subjects who were able may compromise health and behavioral functions. Self- to reverse these effects by extending their sleep on week- reported sleep durations are frequently less than 8 hours ends were able to “liquidate the debt.”3, p. 317 per night. For example, approximately 20% of more than The term sleep debt is usually synonymous with chronic 1.1 million Americans indicated that they slept 6.5 hours sleep restriction because it refers to the increased pressure or less each night.11 Similarly, in polls of 1000 American for sleep that results from an inadequate amount of physi- adults by the National Sleep Foundation, 15% of subjects ologically normal sleep.4 To determine the effects of (aged 18 to 84 years) reported sleeping less than 6 hours chronic sleep loss on a range of neurobehavioral and physi- on weekdays, and 10% reported sleeping less than 6 hours ologic variables, a variety of paradigms have been used, on weekends over the past year.12 Scientific perspectives on including controlled, restricted time in bed for sleep the duration of sleep that defines chronic sleep restriction opportunities in both continuous and distributed sched- have come from a number of theories. ules,5 gradual reductions in sleep duration over time,6 selective deprivation of specific sleep stages,7 and limiting THEORETICAL PERSPECTIVES ON the time in bed to a percentage of the individual’s habitual SLEEP NEED AND SLEEP DEBT time in bed.8 These studies have ranged from 24 hours9 to 8 months6 in length. Basal Sleep Need Many reports published before 1997 concluded that The amount of sleep habitually obtained by an individual chronic sleep restriction in the range commonly experi- is determined by a variety of factors. Epidemiologic and enced by the general population (i.e., sleep durations of experimental studies point to a high between-subjects vari- less than 7 hours per night but greater than 4 hours per ance in sleep duration, influenced by environmental, night) resulted in some increased subjective sleepiness but genetic, and societal factors. Although not clearly defined had little or no effect on cognitive performance capabili- in the literature, the concept of basal sleep need has been ties. Consequently, there was a widely held belief that described as habitual sleep duration in the absence of pre- individuals could “adapt” to chronic reductions in sleep existing sleep debt.13 Sleep restriction has been defined as duration, down to 4 to 5 hours per day. However, nearly the fundamental duration of sleep below which waking all of these reports of adaptation to sleep loss were limited deficits begin to accumulate.14 Given these definitions, the L 67 68 PART I / Section 1 • Normal Sleep and Its Variations basal need for sleep appears to be between 7.5 and 8.5 “optional” sleep. There is, instead, recovery sleep, which hours per day in healthy adult humans. This number was may or may not be optional, although there have very few based on a study in which prior sleep debt was completely studies of the sleep needed to recover from varying degrees eliminated through repeated nights of long-duration sleep of chronic sleep restriction. that stabilized at a mean of 8.17 hours.15 A similar value was obtained from a large-scale dose–response experiment Adaptation to Sleep Restriction on chronic sleep restriction that statistically estimated One popular belief is that subjects may be acutely affected daily sleep need to average 8.16 hours per night to avoid after initial restriction of sleep length and may then be able detrimental effects on waking functions.4 to adapt to the reduced sleep amount, with waking neuro- cognitive functions unaffected further or returned to base- Core Sleep versus Optional Sleep line levels. Although several studies have suggested that In the 1980s, it was proposed that a normal nocturnal sleep this is the case when sleep duration is restricted to approxi- period was composed of two types of sleep relative to mately 4 to 6 hours per night for up to 8 months,6,9 there functional adaptation: core and optional sleep.16,17 The is also evidence indicating that the adaptation is largely initial duration of sleep in the sleep period was referred confined to subjective reports of sleepiness but not objec- to as core, or “obligatory,” sleep, which was posited to tive cognitive performance parameters.4 This suggests that “repair the effects of waking wear and tear on the cere- the presumed adaptation effect is actually a misperception brum.”16, p. 57 Initially, the duration of required core sleep on the part of chronically sleep-restricted people regarding was defined as 4 to 5 hours of sleep per night, depending how sleep restriction has affected their cognitive on the duration of the sleep restriction.16 The duration of capability. core sleep has subsequently been redefined as 6 hours of One factor thought to be important in adaptation to (good-quality, uninterrupted) sleep for most adults.14 chronic sleep restriction is the abruptness of the sleep Additional sleep obtained beyond the period of core sleep curtailment. One study examined the relationship between was considered to be optional, or luxury, sleep, which “fills rate of accumulation of sleep loss, to a total of 8 hours, and the tedious hours of darkness until sunrise.”16, p. 57 This core neurobehavioral performance levels.19 After 1 night of total versus optional theory of sleep need is often presented as sleep deprivation (i.e., a rapid accumulation of 8 hours of analogous to the concept of appetite: Hunger drives one sleep loss), neurobehavioral capabilities were significantly to eat until satiated, but additional food can still be con- reduced. When the accumulation of sleep loss was slower, sumed beyond what the body requires. It is unknown achieved by chronically restricting sleep to 4 hours per whether the so-called optional sleep serves any function. night for 2 nights or 6 hours per night for 4 nights, neu- According to the core sleep theory, only the core portion robehavioral performance deficits were evident, but they of sleep—which is dominated by slow-wave sleep (SWS) were of a smaller magnitude than those following the night and slow-wave activity (SWA) on an electroencephalogram of total sleep loss. A greater degree of neurobehavioral (EEG)—is required to maintain adequate levels of daytime impairment was evident in those subjects restricted to 4 alertness and cognitive functioning.16 The optional sleep hours for 2 nights than in those subjects allowed 6 hours does not contribute to this recovery or maintenance of per night, leading to the conclusion that during the slowest neurobehavioral capability. This theory was strengthened accumulation of sleep debt (i.e., 6 hours per night for 4 by results from a mathematical model of sleep and waking nights), there was evidence of a compensatory adaptive functions (the three-process model) that predicted that mechanism.19 waking neurobehavioral functions were primarily restored It is possible but not scientifically resolved that different during SWS,18 which makes up only a portion of total sleep objective neurobehavioral measures may show different time.