Kleine-Levin syndrome (KLS) is a rare sleep disorder characterized by persistent episodic hypersomnia and cognitive or mood changes. Many patients also experience hyperphagia, hypersexuality and other symptoms. Patients generally experience recurrent episodes of the condition for more than a decade. Individual episodes generally last more than a week but less than a month. The condition greatly affects the personal, professional, and social lives of sufferers, but symptoms spontaneously resolve and seldom cause permanent issues. The severity of symptoms and the course of the disease vary between sufferers. Patients commonly have about 20 episodes over about a decade. KLS is very rare, occurring at a rate of one in a million, which limits research into genetic factors.