Sleep Disorders Physiology

Sleep is an active, clinically important behavior, during which many physiologic processes of the organism change. These processes are temporally organized under cerebral influence. Sleep abnormality may represent a primary disorder of mechanisms regulating sleep, or failure within a specific organ system that manifests in a unique way during sleep. Thus it is important that sleep complaints are not ignored or treated empirically with pharmacologic agents. More often than not, disordered sleep is a symptom of underlying disease.

There are two sleep states, rapid eye movement (REM) and non-REM (NREM) sleep. REM sleep was discovered by Aserinsky and Kleitman in the early 1950s. By waking subjects during REM, they also established that vivid dreaming occurs during this sleep state. Soon thereafter Dement recognized that episodes of REM sleep alternate with NREM sleep, in cycles lasting approximately 90 minutes, throughout the night.

Recordings derived from electroencephalograms (EEG), eye movements (electrooculogram; EOG), and chin tone (electromyogram; EMG) are necessary to distinguish sleep states. Polysomnography is the technique used to record multiple physiologic variables during sleep. In clinical practice several other physiologic measures are typically recorded, including respiratory effort from chest and abdomen, oral and nasal airflow, cardiac rhythm, and electromyographic activity in the anterior tibialis muscles of the legs.

NREM sleep is divided into four sleep stages, numbered I, II, III, and IV. The waking EEG with the eyes closed reveals the characteristic "alpha rhythm," which is a posteriorly predominant 8- to 12-Hz rhythm that attenuates with eye opening. Stage I sleep is characterized by the gradual disappearance of the alpha rhythm, which is replaced by slower, 2- to 7-Hz activity, and some fast 12- to 14-Hz low-voltage activity. Stage II is distinguished by the presence of spindles (bursts of 12- to 14-Hz activity lasting at least 0.5 seconds, with a "spindle" appearance) and K complexes (high-voltage biphasic negative-positive waves best seen at the vertex, usually associated with spindles). Stages III and IV are defined by the presence of high-voltage slow-wave activity of 2 Hz or less. An epoch of 30 seconds is measured: if 20% to 50% of the record shows high-voltage slow activity, it is termed stage III; more than 50% high-voltage slow activity is termed stage IV. Stages III and IV are often described together as slow wave sleep or delta sleep. Delta sleep can be characterized as "deep sleep," because during this stage subjects are difficult to arouse. Detailed dreaming does not occur, but subjects awakened during this sleep stage have reported "thinking."

The normal subject descends in an orderly progression through the four NREM sleep stages. Delta sleep appears 30 to 45 minutes after sleep onset. The first REM period follows this delta sleep, about 70 to 90 minutes after sleep onset.

The polysomnogram during REM shows dramatic changes. There is a sudden decrease in the EMG reading in the chin muscles, which reflects generalized skeletal muscle atonia. Except for the eye muscles and respiratory muscles, the subject is paralyzed. Rapid eye movements occur in phasic bursts. The EEG shows mixed frequencies similar to those of stage I and waking. Characteristic sawtooth waves sometimes occur during eye movements. Respiration and heart rate are irregular.