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Section 23
Understanding NREM and REM Parasomnias

Question 23 | Test | Table of Contents

The parasomnias are conditions in which sleep is disrupted by inappropriate activation, sometimes in the brain centers that control body movements and sometimes in the autonomic nervous system, which governs various physiological and emotional functions. Some parasomnias are characteristic of non-REM (NREM or dreamless) sleep and occur mainly in the first third of the night. Others are typical of REM (rapid eye movement or dreaming) sleep and occur mainly in the last two-thirds of the night. Still others may occur at the transition between sleeping and waking or at any time during the night.

The most familiar parasomnias are sleepwalking (somnambulism) and tooth grinding (bruxism), which are caused by motor activation, and enuresis (bed-wetting), which is caused by autonomic activation of the bladder. Other motor parasomnias are sleep paralysis, rhythmic movement disorder, REM sleep behavior disorder, hypnagogic starts, and nocturnal leg cramps. Autonomic parasomnias include sleep terrors, nightmares, and painful erections.

Motor parasomnias result from the effects of sleep on the brain's motor control system. To fall asleep we must usually change our posture and muscle tone by lying down and relaxing our muscles. As a result, the centers in the brainstem that preserve the waking state are deactivated, and the slow waves of NREM sleep appear on the electroencephalograph (EEG). As NREM sleep deepens through four stages, organized movement gradually becomes more difficult. Several times a night, beginning about 90 minutes after sleep begins, REM periods interrupt slow-wave sleep. During these periods, motor centers are active (dreams are full of motion), but their commands are not obeyed because neurons in the spinal cord are inhibited and almost all muscle activity is suppressed.

The reticular formation of the brain stem, which generates the impulses responsible for major body movements, contains cells that produce several kinds of neurotransmitters, including norepinephfine, serotonin, acetylcholine, and dopamine. Norepinephrine and serotonin neurons become less active in NREM sleep and completely inactive in REM sleep. Neurons that produce acetylcholine are quiescent in NREM sleep but reactivated during REM periods. Nerve circuits using dopamine, which are necessary for all motor control, never become completely quiescent. This may explain why complex movement is never entirely suppressed and can occur even in very deep sleep. The position of the body changes throughout the night, and occasional muscle twitches as well as eye movements occur even during the REM stage.

NREM sleep disorders
The most common motor disorder of NREM sleep is sleepwalking. The movements of sleepwalkers, so comp]ex that they seem almost purposeful, range from manipulation of sheets and nightclothes to excursions out of the bedroom or the house. Somnambulism is most common in the young, and the susceptibility runs strongly in families. The slow waves of NREM sleep persist throughout a sleepwalking episode, and sleepwalkers usually cannot recall it the next morning. They generally return to normal sleep on their own and in their own beds, and rarely harm themselves or anyone else, but there are occasional cases of assault and even homicide, and the courts have granted immunity to the perpetrators. Drugs are rarely necessary to prevent sleepwalking, although a short-acting benzodiazepine like triazolam (Halcion) may be useful in severe and persistent cases. Before prescribing any drugs, clinicians should be certain that the symptoms are not caused by complex partial epileptic seizures or by REM sleep behavior disorder (described below).

Rhythmic movements, especially head rolling, rocking, and cries, are common during NREM sleep in childhood, especially in early infancy. The movements, probably a soothing mechanism, run strongly in families and occur far more often in boys than in girls. The sleeper is usually half awake and, if old enough, responds to a command to go back to bed. The movements almost always subside with age and rarely require treatment unless there is a risk of head injury.

Nocturnal leg cramps, which occur mainly in the elderly, may be intensely painful and cause partial or complete awakening. Women are more vulnerable. Pregnancy, the use of oral contraceptives, diabetes, and prolonged vigorous exercise raise the risk, possibly by depleting calcium and potassium. Hormone and mineral replacement therapy may be helpful.

The brain strongly resists arousal from NREM sleep. This sleep inertia may lead to confusion, anxiety, and involuntary movements when a person is awakened early in the night. Sleep deprivation raises the risk. For example, when mathematics students are kept awake for a long time, allowed to fall asleep, and then aroused after a short time, they cannot even subtract two-digit numbers.

Young children, who have the deepest NREM sleep and the greatest sleep inertia, are especially susceptible to confusional arousal and night terrors, as well as to sleepwalking. The risk of all NREM sleep disorders declines with maturity and disappears almost entirely by age 40, along with stages III and IV, the deepest phases of NREM sleep. Since the hormones that promote growth and sexual maturation are released during this stage of the sleep cycle, its disorders may be a price we pay for crucial processes of development.

REM sleep disorders
In REM sleep the amygdala and other brain regions that mediate anxiety and fear are strongly activated, while the dorsolateral prefrontal cortex, which normally moderates the expression of these emotions during waking hours, becomes relatively inactive. As a result, anxiety is the most common emotion in dreams and sometimes becomes uncontrollable. Some people dread going to sleep because of their REM sleep anxiety dreams, better known as nightmares. When nightmares persistently lead to awakening and cause sleep loss, the condition is known as REM sleep interruption insomnia.

Children, again, are especially susceptible, because they have so much REM sleep. Rare in adult life, nightmares occur in 50% of children at the