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Five distinct sleep stages can be measured by polysomnography: rapid eye movement (REM) sleep and four stages of non-rapid eye movement (NREM) sleep (stages 1, 2, 3, and 4). Stage 1 NREM sleep is a transition from wakefulness to sleep and occupies about 5% of time spent asleep in healthy adults. Stage 2 NREM sleep, which is characterized by specific EEG waveforms (sleep spindles and K complexes), occupies about 50% of time spent asleep. Stages 3 and 4 NREM sleep (also known collectively as slow-wave sleep) are the deepest levels of sleep and occupy about 10%-20% of sleep time. REM sleep, during which the majority of typical storylike dreams occur, occupies about 20%-25% of total sleep.

These sleep stages have a characteristic temporal organization across the night. NREM stages 3 and 4 tend to occur in the first one-third to one-half of the night and increase in duration in response to sleep deprivation. REM sleep occurs cyclically throughout the night, alternating with NREM sleep about every 80-100 minutes. REM sleep periods increase in duration toward the morning. Human sleep also varies characteristically across the life span. After relative stability with large amounts of slow-wave sleep in childhood and early adolescence, sleep continuity and depth deteriorate across the adult age range. This deterioration is reflected by increased wakefulness and stage 1 sleep and decreased stages 3 and 4 sleep. Because of this, age must be considered in the diagnosis of a Sleep Disorder in any individual.

Polysomnography is the monitoring of multiple electrophysiological parameters during sleep and generally includes measurement of EEG activity, electrooculographic activity, and electromyographic activity. Additional polysomnographic measures may include oral or nasal airflow, respiratory effort, chest and abdominal wall movement, oxyhemoglobin saturation, or exhaled carbon dioxide concentration; these measures are used to monitor respiration during sleep and to detect the presence and severity of sleep apnea. Measurement of peripheral electromyographic activity may be used to detect abnormal movements during sleep. Most polysomnographic studies are conducted during the person's usual sleeping hours - that is, at night. However, daytime polysomnographic studies also are used to quantify daytime sleepiness. The most common daytime procedure is the Multiple Sleep Latency Test (MSLT), in which the individual is instructed to lie down in a dark room and not resist falling asleep; this protocol is repeated five times during the day. Sleep latency (the amount of time required to fall asleep) is measured on each trial and is used as an index of physiological sleepiness. The converse of the MSLT is also used: In the Maintenance of Wakefulness Test (MWT), the individual is placed in a quiet, dimly lit room and instructed to remain awake; this protocol is repeated several times during the day. Again, sleep latency is measured, but it is used here as an index of the individual's ability to maintain wakefulness.

Standard terminology for polysomnographic measures is used throughout the text in this section. Sleep continuity refers to the overall balance of sleep and wakefulness during a night of sleep. "Better" sleep continuity indicates consolidated sleep with little wakefulness; "worse" sleep continuity indicates disrupted sleep with more wakefulness. Specific sleep continuity measures include sleep latency - -the amount of time required to fall asleep (expressed in minutes); intermittent wakefulness - the amount of awake time after initial sleep onset (expressed in minutes); and sleep efficiency - the ratio of actual time spent asleep to time spent in bed (expressed as a percentage, with higher numbers indicating better sleep continuity). Sleep architecture refers to the amount and distribution of specific sleep stages. Sleep architecture measures include absolute amounts of REM sleep and each NREM sleep stage (in minutes), relative amount of REM sleep and NREM sleep stages (expressed as a percentage of total sleep time), and latency between sleep onset and the first REM period (REM latency).

The text for each of the Sleep Disorders contains a section describing its relationship to corresponding disorders in the The International Classification of Sleep Disorders: (ICSD) Diagnostic and Coding Manual, published in 1990 by the American Sleep Disorders Association.

For instance, several studies have shown that persistent complaints of insomnia are a risk factor for the subsequent development of depression, anxiety disorders, and substance abuse disorders (Breslau et al. 1996; Chang et al. 1997; Ford and Kamerow 1989; Livingston et al. 1993; Weissman et al. 1997). These relationships hold both for young adult and for elderly populations. Furthermore, surveys of patient diagnoses at sleep disorders centers reveal that psychiatric disorders are very common causes of complaints of chronic insomnia.

In a survey of nearly 1,300 insomnia patients at sleep disorders centers, insomnia related to psychiatric disorders accounted for 35% - the largest single group - of chronic insomnia patients (Coleman et al. 1982). The DSM-IV Field Trial of sleep disorder diagnoses demonstrated a similar finding (Buysse et al. 1994). Forty-four percent of patients with chronic insomnia had a primary clinical diagnosis of insomnia related to another mental disorder, and another 30% received insomnia as a secondary diagnosis.

Perhaps equally important, patients with sleep disorders often present with signs and symptoms of psychiatric disorders. For instance, patients with obstructive sleep apnea syndrome often present with symptoms of anergia, amotivation, poor concentration and memory, and sleep disturbance as primary manifestations of their sleep disorder. Given the physical and psychosocial toll of sleep apnea, secondary depression often occurs as well. Patients with narcolepsy, who often have hallucinatory experiences near the onset or end of sleep, may be inappropriately labeled as psychotic. Patients with parasomnias may also come to psychiatric attention because of their unusual behavior during sleep. Therefore, whether psychiatric disturbances cause sleep problems or sleep disorders cause psychiatric symptoms, mental health professionals need to develop skills in the evaluation and treatment of sleep disorders.