Nocturnal myoclonus (or periodic movements in sleep) consists of stereotyped sleep-related movements of the lower limbs and occasionally also upper limbs, ranging from simple dorsiflexion of the big toe and foot to a triple flexion of the entire leg. It is characterized by a typical periodicity, often occurring in association with sleep arousal phenomena. As an isolated finding (essential nocturnal myoclonus), it represents a paraphysiological phenomenon, also found in normal subjects and developing with advancing age. On the other hand, symptomatic nocturnal myoclonus is typically associated with restless legs syndrome; in this condition, it is usually severe and present also during wakefulness. The exact site of origin of nocturnal myoclonus is unknown. It is almost certainly a subcortical phenomenon, probably modulated in its periodicity by reticular influences. It has frequently been confused with, and should be clearly differentiated from, other normal jerking movements of sleep, such as partial myoclonic jerks and massive myoclonic jerks, or sleep starts. Other abnormal movements that may be confused with nocturnal myoclonus are the startles of hyperekplexia, the syndrome of painful legs and moving toes, nocturnal leg cramps, and the numerous varieties of epileptic myoclonus.