Disorders of sleep belong to the most frequent troubles. A lot of different factors may contribute to the pathogenesis of sleep disorders. Modern sleep research has opened important views into the structure of sleep and the pathophysiology of sleep disorders. In a narrower sense sleep disorders mean a deficiency of sleep (hyposomnia). This may concern the induction of sleep or its maintenance. Sleep disorders without underlying disease may be differentiated from those caused by different diseases. Hypersomnia may occur as a symptom of various cerebral dysfunctions and processes, as an episodic disorder (Kleine-Levin-syndrome) or as a part of the Pickwick-syndrome. In narcolepsy characteristic symptoms are sleep attacks, kataplexy (emotionally induced loss of muscle tone) and transient pareses during awakening. Disturbances linked with sleep are snoring, somnambulism, speaking and grinding of the teeth during sleep and nocturnal enuresis. They may constitute idiopathic disorders. On the other hand, there may be similarities between epileptic phenomena and these disorders. Some epilepsies demonstrate relations to the rhythm of sleep and wakefulness which become evident in the manifestation of seizures and epileptiform EEG activity at certain stages of vigilance.