Parasomnias are disorders of partial arousal or disorders that interfere with sleep stage transitions. Abnormal events occur during sleep. The International Classification of Sleep Disorders lists four subcategories of parasomnia disorders: arousal, sleep-wake transition, REM sleep, and other.

These are disorders of partial arousal (being awakened):

• Confusional arousals. Confusion during and following arousals from sleep, most typically from deep sleep in the first part of the night.

• Sleepwalking. Walking during sleep. About 1 to 15% of the population sleepwalks. It is more common in children than in adolescents and adults.

• Sleep terrors. A sudden arousal from slow wave sleep with a scream or cry, accompanied by symptoms of intense fear.

These parasomnias are disorders that interfere with sleep stage transitions:

• Rhythmic movement disorder. Repetitive bodyrocking and headrolling of infants and toddlers, as they are falling asleep and just staring to sleep.

• Sleep starts. Very common sudden brief contractions of the legs (and sometimes arms and head) just as sleep is beginning. In rare cases, extreme sleep starts can cause sleep onset difficulties.

• Sleep talking. Talking or making sounds during sleep without the sleeper being aware. This is common; in rare cases, the noise can be a major annoyance to a sleep partner.

• Nocturnal leg cramps. A painful muscle tightness in the calf or occasionally in the foot during sleep. More prevalent in the elderly and after vigorous exercise.

These parasomnias occur during the rapid-eye movement (REM) stage of sleep. Common examples include:

• Nightmares. Frightening dreams that usually awaken the sleeper from REM sleep. Half of adults have occasional nightmares; 1% of adults have nightmares once or more per week.

• Sleep paralysis. Inability to move at beginning of sleep or after awakening.

• REM sleep behavior disorder. A disorder of unusual movements and or verbalization related to REM sleep, at times associated with injury to the sleeper or spouse.

Other parasomnias include the following:

• Sleep-related abnormal swallowing syndrome. Inadequate swallowing of saliva, causing coughing, choking and brief arousals or awakenings from sleep.

• Nocturnal paroxysmal dystonia. Nighttime involuntary muscle contractions during non-REM sleep. (During REM or dreaming sleep, the body can move involuntarily.) The contractions can cause twisting movements, abnormal postures, limb movements, fidgeting and/or writhing. The attacks usually last 30 to 45 seconds. They can occur as often as five times a night or as few as five per year.

• Congenital Central Hypoventilation Syndrome. A rare condition with slow and shallow breathing, resulting in an increased level of carbon dioxide in the blood. CCHS is unexplained by primary pulmonary disease or ventilatory muscle weakness.

• Infant sleep apnea. Central or obstructive sleep apneas in infants. Pre-term infants have an increased risk.

• Sudden Infant Death Syndrome (SIDS). The sudden death of an infant under one year of age which remains unexplained after a thorough case investigation. Very little is known about the causes, and there is no method for absolute prevention. A number of websites have information and links about how to lessen the risk of SIDS.

The cause and treatment of parasomnias varies depending on an individual's situation and case history.