The function of sleep remains unknown. Although experience suggests that sleep is a restorative process that results in the person feeling rested, researchers have not yet been able to establish the physiological reasons for this, despite many years of investigation. Various theories about the purpose of sleep have been proposed, but none has been proven conclusively.

Sleep involves many complex physiological processes in virtually all body systems. Although sleep is considered a time of inactivity and rest, many physiological events occur throughout the sleep cycles, such as changes in heart rate, blood pressure, and respiration. Growth hormone and prolactin levels increase, and cortisol and thyrotropin (TSH) levels decrease. These normal events may be altered markedly in conditions of disturbed sleep, such as sleep deprivation.

12.2.1 Stages of Sleep

There are two types of sleep: non-rapid eye movement (NREM) and rapid eye movement (REM). Both are marked by characteristic physiologic changes. Each stage is associated with characteristic patterns of EEG activity.

NREM sleep includes stages 1, 2, 3, and 4. During stage 1, the person is roused most easily, and as the sleep stages progress it becomes more difficult to awaken the person. Stages 3 and 4 are referred to as “deep sleep” and people perceive these stages as high-quality sleep. NREM has been described as a relatively inactive yet actively regulated brain in a movable body. During NREM sleep, heart rate, blood pressure, and respiratory rate generally decrease.

Rapid eye movements and vivid dreams characterize REM sleep, which is also associated with great electrical brain activation and loss of muscle tension, or atonia. In contrast to NREM, heart rate and blood pressure are more variable and may increase.

12.2.2 Sleep Regulating Processes

The two primary physiologic processes that regulate sleep cycles are the circadian rhythm and the homeostatic process. The circadian rhythm is a pacemaker or biological clock that regulates the daily patterning of sleep. Circadian characteristics result in sleep and wake occurring during predictable times of the day and night. The circadian pacemaker stimulates changes in the daily patterns of sleep and wakefulness that are endogenous to the organism. The changes occur in an approximately 24-hour rhythm, but exogenous factors also influence the timing of these changes. An example is environmental changes in light and dark. Sleep disturbances may result from changes in exogenous factors, such as night work or changes in time zones associated with air travel.

Homeostasis describes the coordinated physiological processes that maintain most steady states in an organism. In humans, the need for sleep increases as the time awake increases. Prolonged periods of wakefulness result in decreased alertness, increased sleepiness, and greater amounts of slow-wave (NREM stage 4) sleep. Once sleep occurs, homeostasis, or the steady state, is accomplished and the accumulated “sleep debt” is replenished and patients awake feeling rested.

12.2.3 Normal Age-related Changes in Sleep and Wakefulness

Newborns spend nearly 50% of total sleep time in REM sleep. Because infants may sleep up to 16 hour a day, they may spend 8 hours in REM sleep. Children display a polyphasic sleep–wake pattern, with short bouts of sleep and wakefulness throughout the 24-hour day, until several months of age when they eventually sleep through the night. Daytime napping, however, often persists until the age of 4–6 years.

Maximal “depth” of sleep may occur during the prepubertal period, when children are often difficult to awaken at night. Adolescents often still need at least 10 hours of sleep. Yet, during adolescence, stages 3 and 4 sleep decline and daytime sleepiness increases, partially in association with the normal Tanner stages of pubertal development. Teenagers are also phase-delayed, which means that they may not get sleepy until the early morning hours (e.g., 2–3 a.m.) and do not naturally wake up until the later morning hours. Early school start times and social pressures may produce mild sleep deprivation during weekdays, with some “catch-up” occurring on weekends.

As adults enter middle age and old age, sleep often becomes more shallow, fragmented, and variable in duration and circadian timing compared with that of young adults. Stages 1 and 2 and the amount of time spent awake during the night tend to increase; REM latency and slow-wave sleep decline. Most age-related sleep changes occur in early and mid-years of the human lifespan, and in healthy older adults sleep remains fairly constant between the ages of 60 and 90 years.

Daytime sleepiness and napping usually increase with age, often as a function of disturbed nocturnal sleep. Adults in the sixth decade and older may experience advanced sleep phase syndrome, which involves falling asleep early in the evening and awakening very early in the morning. Although many sleep experts believe this finding to be a normal developmental pattern, advanced sleep phase syndrome may interfere with preferred work and leisure activities. Many clinicians believe that it is normal for older adults to report more sleep disturbances than young and middle adults. The belief that sleep disturbance is a normal part of aging is particularly common among older adults and may prevent them from seeking help with genuine sleep problems. Studies suggest that observed higher rates of sleep disturbance among older adults are associated with medical or psychiatric illness. For this reason, clinicians should be especially cognizant of performing sleep assessments with older adult patients and assisting them with interventions to improve sleep.

Sleep hygiene includes strategies to manipulate environmental conditions and personal behaviors to support effective sleep. Just as human beings need a balanced diet to promote and maintain health, they need habits and a structure to promote effective sleep. The general principles of sleep hygiene include maintaining a regular sleep–wake schedule, environmental modifications, and reductions in sources of arousals.

Although numerous strategies have been recommended, no universal consensus has been reached about the best strategies to use, and more research is needed. Most sleep specialists teach patients to maintain a regular sleep schedule on weekdays and weekends, to modify the sleeping environment to reduce noise and light, to maintain a comfortable environmental temperature, and to sleep in a comfortable bed. Clinicians should discourage patients from eating a large meal or spicy foods near bedtime; conversely, they instruct patients to avoid going to bed on an empty stomach. Many sleep specialists recommend that patients consume a light snack of carbohydrate-containing foods or warm milk prior to bedtime and avoid caffeine within 6 hours of bedtime.

Sleep disorders include dyssomnias and parasomnias. Dysomnias are abnormalities in the amount, quality, or timing of sleep and include narcolepsy, breathing-related sleep disorders, periodic limb movement disorder, and insomnia. Parasomnias are abnormal behavioral or physiologic events associated with sleep. Parasomnias include problems such as sleepwalking and tooth grinding and are not discussed in this chapter.

12.4.1 Insomnia