Parasomnias are undesirable or unusual physical, behavioural, experiential or physiological events that occur during sleep, during transitions between sleep and wakefulness or around partial arousals from sleep. They encompass several distinct phenomena rather than constituting one disease. Examples include sleepwalking, sleep terrors, certain abnormal behaviours during REM sleep and other events involving movement, vocalisation, autonomic activation or unusual experiences. Classification depends partly on the sleep state from which the event arises, with disorders associated with non-REM sleep differing in important ways from REM-related parasomnias.
Non-REM parasomnias commonly arise from incomplete arousals out of deeper non-REM sleep, particularly during the first part of the night when slow-wave sleep is more prominent. The person may appear awake but have limited awareness and impaired responsiveness, and later recall can be minimal or absent. Sleepwalking and sleep terrors are characteristic examples. These events can involve complex behaviour despite incomplete cortical arousal. Factors that increase sleep pressure, disrupt sleep continuity or promote partial arousals can influence their occurrence.
REM-related parasomnias have a different physiological basis because REM sleep normally includes profound skeletal-muscle atonia that prevents most dream-related movements from being acted out. In REM sleep behaviour disorder, this normal motor inhibition is impaired, allowing vocalisation or complex movements associated with dream content. This condition is particularly important clinically because persistent REM sleep behaviour disorder can be associated with neurological disease in some older adults. It is distinct from ordinary nightmares because the defining feature involves abnormal motor or behavioural expression during REM sleep.
Parasomnias can be influenced by sleep deprivation, irregular schedules, stress, illness, medications, alcohol and other factors that alter sleep architecture or arousal thresholds. Fever and environmental disruption can also contribute to some non-REM parasomnias, particularly in children. Certain sleep disorders can increase the frequency of abnormal arousals. The relationship between triggers and parasomnias is not necessarily deterministic: a person may have an underlying susceptibility that becomes apparent only under particular circumstances.
Parasomnias occur across the lifespan, and many childhood non-REM parasomnias become less frequent as the nervous system matures. Sleepwalking and sleep terrors can occur in otherwise healthy children and do not automatically indicate psychiatric or neurological disease. Persistent, severe or newly emerging episodes in adulthood warrant greater attention to possible underlying sleep disorders, medications or neurological conditions. Age, timing of episodes, level of awareness and memory after the event are useful clues when distinguishing among different parasomnia types.
Clinical assessment begins with a detailed account of the event, including timing, behaviour, vocalisations, awareness, responsiveness, memory and potential triggers. Information from a bed partner, parent or other observer can be particularly valuable because the affected person may have little or no recollection. Video recordings can sometimes help document unusual episodes. Polysomnography with audiovisual monitoring may be indicated when the diagnosis is uncertain, events are potentially dangerous, seizures are a differential diagnosis or an underlying sleep disorder is suspected.
Safety is an important component of managing parasomnias because complex behaviour can occasionally lead to falls, leaving the bedroom, accidental injury or harm to another person. Management may include improving sleep regularity, reducing sleep deprivation and addressing coexisting sleep disorders. The environment may need modification when episodes create physical hazards. Treatment is specific to the type and severity of parasomnia, and medication is not automatically necessary. The aim is to identify and reduce precipitating factors while addressing clinically significant risks.
Parasomnias are frequently confused with nightmares, ordinary dreaming, insomnia or epileptic events. Dreaming itself is normal, whereas a parasomnia involves an abnormal or undesirable event associated with sleep or sleep transitions. Some parasomnias can resemble nocturnal seizures, making careful assessment important when episodes are stereotyped, unusually violent, frequent or atypical. The broad term therefore should not be treated as a diagnosis with one mechanism: its usefulness lies in grouping sleep-related behaviours and experiences that must subsequently be classified according to their physiological and clinical characteristics.
