Sleep disorders

The Stuff of Sleep

Sleep appears to be a passive and restful time when the brain is less active. In fact, this state actually involves a highly active and well-scripted interplay of brain circuits to produce the stages of sleeping.

The stages of sleep were discovered in the 1950s in experiments examining the human brain waves or electroencephalogram (EEG) during sleep. Researchers also measured movements of the eyes and the limbs during sleep. They found that over the course of the first hour or so of sleep each night, the brain progresses through a series of stages during which the brain waves progressively slow down. The period of slow wave sleep is accompanied by relaxation of the muscles and the eyes. Heart rate, blood pressure and body temperature all fall. If awakened at this time, most people recall only a feeling or image, not an active dream.

Over the next half hour or so, the brain emerges from the deep slow wave sleep as the EEG waves become progressively faster. Similar to during waking, rapid eye movements emerge, but the body’s muscles become almost completely paralyzed (only the muscles that allow breathing remain active). This state is often called rapid eye movement (REM) sleep. During REM sleep, there is active dreaming. Heart rate, blood pressure and body temperature become much more variable. The first REM period usually lasts ten to 15 minutes.

Over the course of the night, these alternative cycles of slow wave and REM sleep alternate, with the slow wave sleep becoming less deep, and the REM periods more prolonged, until waking occurs.

Over the course of a lifetime, the pattern of sleep cycles changes. Infants sleep up to 18 hours per day, and they spend much more time in deep slow wave sleep. As children mature, they spend less time asleep, and less time in deep slow wave sleep. Older adults may sleep only six to seven hours per night, often complain of early wakening that they cannot avoid, and spend very little time in slow wave sleep.

The most common sleep disorder, and the one most people are familiar with, is insomnia. Some people have difficulty falling asleep initially, but other people fall asleep, and then awaken part way through the night, and cannot fall asleep again. Although there are a variety of short-acting sedatives and sedating antidepressant drugs available to help, none of these produces a truly natural and restful sleep state because they tend to suppress the deeper stages of slow wave sleep.

Excessive daytime sleepiness may have many causes. The most common are disorders that disrupt sleep and result in inadequate amounts of sleep, particularly the deeper stages. These are usually diagnosed in the sleep laboratory. Here, the EEG, eye movements and muscle tone are monitored electrically as the individual sleeps. In addition, the heart, breathing, and oxygen content of the blood can be monitored.

Obstructive sleep apnea causes the airway muscles in the throat to collapse as sleep deepens. This prevents breathing, which causes arousal, and prevents the sufferer from entering the deeper stages of slow wave sleep. This condition can also cause high blood pressure and may increase the risk of heart attack. There is also an increased risk of daytime accident, especially automobile accidents, which may prevent driving. Treatment is complex and may include a variety of attempts to reduce airway collapse during sleep. While simple things like losing weight, avoiding alcohol and sedating drugs prior to sleep, and avoiding sleeping on one’s back can sometimes help, most people with sleep apnea require positive airway pressure to keep the airway open. This can be provided by fitting a small mask over the nose that provides an air stream under pressure during sleep. In some cases, surgery is needed to correct the airway anatomy.

Periodic limb movements of sleep are intermittent jerks of the legs or arms, which occur as the individual enters slow wave sleep, and can cause arousal from sleep. Other people have episodes in which their muscles fail to be paralyzed during REM sleep, and they act out their dreams. This REM behavior disorder can also be very disruptive to a normal nights’ sleep. Both disorders are more common in people with Parkinson’s disease, and both can be treated with drugs that treat Parkinson’s, or with an anti-epileptic drug called clonazepam.

Narcolepsy is a relatively uncommon condition (one case per 2,500 people) in which the switching mechanism for REM sleep does not work properly. Narcoleptics have sleep attacks during the day, in which they suddenly fall asleep. This is socially disruptive, as well as dangerous, for example, if they are driving. They tend to enter REM sleep very quickly as well, and may even enter a dreaming state while still awake, a condition known as hypnagogic hallucinations. They also have attacks during which they lose muscle tone, similar to what occurs during REM sleep, but while they are awake. Often, this occurs while they are falling asleep or just waking up, but attacks of paralysis known as cataplexy can be triggered by an emotional experience or even hearing a funny joke.

Recently, insights into the mechanism of narcolepsy have given major insights into the processes that control these mysterious transitions between waking, slow wave and REM sleep states. (From Brain Facts: a Primer on the Brain and Nervous System, 2002)

Дата добавления: 2014-12-29