Disturbed sleep is among the most frequent health complaints physicians encounter. More than one-half of adults in the United States experience at least intermittent sleep disturbance. For most, it is an occasional night of poor sleep or daytime sleepiness. However, the Institute of Medicine has estimated that 50–70 million Americans suffer from a chronic disorder of sleep and wakefulness, which can lead to serious impairment of daytime functioning. In addition, such problems may contribute to or exacerbate medical or psychiatric conditions. Thirty years ago, many such complaints were treated with hypnotic medications without further diagnostic evaluation. Since then, a distinct class of sleep and arousal disorders has been identified.
Given the opportunity, most adults will sleep 7–8 h per night, although the timing, duration, and internal structure of sleep vary among healthy individuals and as a function of age. At the extremes, infants and the elderly have frequent interruptions of sleep. In the United States, adults tend to have one consolidated sleep episode per day, although in some cultures sleep may be divided into a mid-afternoon nap and a shortened night sleep. Two principal neural systems govern the expression of the sleep and wakefulness states within the daily cycle. The first potentiates sleep in proportion to the duration of wakefulness (the "sleep homeostat"), while the second rhythmically modulates sleep and wakefulness tendencies at appropriate phases of the 24-h day (the circadian clock). Intrinsic abnormalities in the function of either of these systems, or extrinsic disturbances (environmental, drug- or illness-related) that supersede their normal expression, can lead to clinically recognizable sleep disorders.
States and Stages of Sleep
States and stages of human sleep are defined on the basis of characteristic patterns in the electroencephalogram (EEG), the electrooculogram (EOG—a measure of eye-movement activity), and the surface electromyogram (EMG) measured on the chin and neck. The continuous recording of this array of electrophysiologic parameters to define sleep and wakefulness is termed polysomnography.
Polysomnographic profiles define two states of sleep: (1) rapid-eye-movement (REM) sleep and (2) non-rapid-eye-movement (NREM) sleep. NREM sleep is further subdivided into three stages, characterized by increasing arousal threshold and slowing of the cortical EEG. REM sleep is characterized by a low-amplitude, mixed-frequency EEG similar to that of NREM stage N1 sleep. The EOG shows bursts of REM similar to those seen during eyes-open wakefulness. Chin EMG activity is absent, reflecting the brainstem-mediated muscle atonia that is characteristic of that state.
Organization of Human Sleep
Normal nocturnal sleep in adults displays a consistent organization from night to night (Fig. 27-1). After sleep onset, sleep usually progresses through NREM stages N1–N3 sleep within 45–60 min. Slow-wave sleep (NREM stage N3 sleep) predominates in the first third of the night and comprises 15–25% of total nocturnal sleep time in young adults. The percentage of slow-wave sleep is influenced by several factors, most notably age (see ...