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Thirty-five percent of adults in the United States experience sleep-related symptoms over the course of a year, making insomnia and daytime sleepiness among the most common symptoms seen in medical practice. Ten to fifteen percent of these adults will suffer chronic insomnia and will be frequent visitors to health care providers. Sleep disorders also pose a major public health threat. Chronic insomnia is associated with doubling of mortality rate over time. Sleep deprivation increases risk for chronic disease, including hypertension, diabetes and depression. Daytime sleepiness impairs work performance and increases the risk of industrial and motor vehicle accidents. Sleep loss due to sleep-related breathing problems leads to profound sleepiness and life-threatening cardiovascular and pulmonary diseases. Sleep medications themselves carry morbidity such as falls, daytime anxiety, and worsened sleep apnea.

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Normal Sleep Physiology

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Adulthood

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The sleep–wake cycle is a complex electrophysiologic process consisting of alternating periods of wakefulness, rapid eye movement (REM) sleep, and non-REM sleep. Each of these periods has a characteristic electroencephalogram (EEG), peripheral muscle, and autonomic nervous system pattern that can be documented by polysomnographic (PSG) recording in hospital-based sleep laboratories, although newer technology allows in-home recordings. PSG allows clinicians to make specific diagnoses based on electrophysiologic monitoring of EEG, electrooculogram, electromyogram, nasal airflow, ear oximetry, and electrocardiogram.

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Sleep has a structure, or architecture, that consists of four stages of non-REM and REM sleep cycles. The wake EEG contains low-voltage, high-frequency waveforms that become dominated by alpha waveforms (8–12 cps) as a person becomes drowsy. Stage 1 sleep is defined by the disappearance of the alpha pattern, the establishment of theta waveforms (2–7 cps) and slow, rolling eye movements. Stage 2 is defined by the appearance of low-frequency, high-amplitude discharges (K complexes) and brief high-frequency (12–14 cps), variable-amplitude discharges (sleep spindles) on a background of theta waveforms similar to stage 1. The emergence of slow waves (high-amplitude, low-frequency [0.5–2 cps] delta waveforms) heralds stage 3 sleep, when they make up at least 20% of sleep time, and stage 4 sleep when they comprise more than 50% of sleep time. These two stages are known as the "deep stages" of sleep, because they are associated with high-arousal thresholds. REM sleep is a distinct state of sleep characterized by wake-pattern EEG, skeletal muscle paralysis, and rapid, conjugate eye movements.

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With the initiation of sleep, the healthy adult will descend through the non-REM stages within 45–60 minutes before beginning the first REM cycle, which tends to be brief. As the night progresses, less time is spent in slow-wave sleep and REM cycle duration increases, eventually comprising 20–25% of total sleep time. The non-REM/REM cycle typically lasts 90–110 minutes, with about four complete cycles per night.

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The timing and duration of sleep are controlled by many factors. Although most adults have some control over when to go to sleep and when to wake up, they have less control over ...

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