Kleine-Levin syndrome: A rare condition that is characterized by periods of excessive drowsiness and sleep, lasting up to 20 hours per day. Affected individuals usually have excessive food intake and may be confused, irritable, and lethargic. Some patients have hallucinations or sexual disinhibition. Most patients with Kleine-Levin syndrome are adolescent males. The cause is unknown. Most cases resolve over time without treatment. Also known as Kleine-Levin hibernation syndrome.