“Nightmares are frightening dreams that usually awaken the sleeper from REM sleep.” Because nightmares occur during REM sleep, the subject arouses easily and has detailed dream recall. Episodes typically occur later in the night, when REM is more predominant. Autonomic activation and confusion are not characteristic features of the nightmare. Commonly, the episodes are recurrent in the same night during each REM period. Nightmares occur normally in childhood. In adults, recurrent nightmares may represent underlying psychiatric disorders, especially anxiety and depression, or REM rebound secondary to drug withdrawal. Sometimes, recurrent nightmares seem to be induced by physiologic disruptions occurring during REM sleep, such as sleep-disordered breathing or arrhythmia.

“Sleep paralysis consists of a period of inability to perform voluntary movements either at sleep onset (hypnogogic or predormital form) or upon awakening either during the night or in the morning (hypnopompic or postdormital form).” Sleep paralysis represents a partial arousal or entry into REM sleep such that the atonia of REM sleep is present but the cerebral cortex is awake. This is a common symptom of narcolepsy (discussed earlier), especially at sleep onset. Normal subjects may have occasional episodes of sleep paralysis on awakening. A familial disorder characterized by sleep-onset sleep paralysis without other features of narcolepsy has been described. Sleep paralysis is particularly common in patients taking REM-modifying drugs, especially MAO inhibitors.

“REM sleep-related sinus arrest is a cardiac rhythm disorder that is characterized by sinus arrest during REM sleep in otherwise healthy individuals.” Asystole may be prolonged, lasting as long as 9 seconds. This arrhythmia responds to atropinergic drugs. Most patients have been treated with artificial pacemakers, because the natural history of the disorder and the risk of sudden death are unknown.

“REM sleep behavior disorder is characterized by the intermittent loss of REM sleep electromyographic (EMG) atonia and by the appearance of elaborate motor activity associated with dream mentation.” Lack of the usual atonia of REM sleep allows the patient to enact dreams. Patients typically run, punch, kick, or leap from bed. Vocalization consistent with the dream activity is usual. This disorder predominantly affects men in late middle age. Forty percent of cases have been associated with neurologic disease, most commonly dementing illnesses of unknown cause, as well as olivopontocerebellar degeneration, Parkinson’s disease, and subarachnoid hemorrhage. A familial predisposition may exist. Diagnosis is made by polysomnography, which shows excessive EMG tone during REM associated with dream enactment. Treatment with a nightly dose of clonazepam is highly effective.