The bizarre and often dangerous parasomnias such as sleepwalking and sleep terrors have been historically regarded as symptoms of psychiatric illness. Parasomnias are undesirable physical, experiential, or behavioral phenomena that occur exclusively during sleep or are exacerbated by the sleeping state. The designation "disorders of arousal", first coined, has reappeared as a category of non-rapid eye movement (NREM) parasomnias and includes confusional arousals, sleepwalking, and sleep terrors. Dissociative disorders may also occur during the sleep period, typically in individuals suffering daytime syndromes such as dissociative identity disorder (DID), psychogenic amnesia, and dissociative disorder not otherwise specified (NOS). Sexsomnia has been classified in the ICSD-2 within the group of parasomnias named "disorders of arousal", with the designation of "sleep-related abnormal sexual behaviors" being a variant of confusional arousals. Some primary psychiatric disorders may include symptoms that occur prominently or exclusively in association with the sleep period.