Thirty-five percent of adults experience sleep-related symptoms over the course of a year, making them among the most common symptoms seen in medical practice. Ten to fifteen percent of adults suffer chronic insomnia and account for the largest proportion of patients seeing health care providers for sleep-related problems. Insomnia increases risk for other chronic diseases, including hypertension, diabetes, depression, and dementia. 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 deprivation is known to increase risk for depression and dementia. Sleep medications themselves carry morbidity such as falls, daytime anxiety, and worsened sleep apnea. In this chapter, we review normal sleep, disorders of both sleep and wakefulness, and a logical clinical approach to these disorders.
SLEEP THROUGH THE LIFE CYCLE
Sleep has a structure, or architecture, that consists of nonrapid eye movement (non-REM) and REM sleep cycles. The wake electroencephalogram (EEG) contains low-voltage, high-frequency waveforms that become dominated by alpha waveforms (8–12 cps) as a person becomes drowsy. There are four stages of non-REM sleep. Stage 1 is defined by the disappearance of the alpha pattern, the appearance of slower theta waveforms (2–7 cps) along with 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 majority of sleep time is spent in stage 2 non-REM. 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. The central nervous system (CNS) is predominantly active during REM sleep, and this is when a majority of dreams occur.
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.
The timing and duration of sleep are controlled by many factors. Although most adults have some control over when they go to bed and when they get up, they have less control over how much sleep they need, and the quality of the sleep they get. Although stimulants such as caffeine and habituation to a state of chronic fatigue ...