A French study of 108 patients found a median duration of 13 years, but a review of 186 cases found a median duration of 8 years. Unusually young or old patients and those who experience hypersexuality tend to have a more severe course. Patients who initially have frequent attacks generally see the disease cease earlier than others. The condition spontaneously resolves, and the patient is considered to be cured if there have been no symptoms for six years. Epidemiology
Population-based studies of KLS have not been performed. Its prevalence is about 1 case per million people. In France, KLS has a prevalence of 1.5 per million people. It occurs most frequently among Jews in the US and Israel. First-degree relatives of people who have the syndrome are much more likely than the general population to have it, although only in about one percent of cases do family members contract it. About 70 to 90 percent of patients are male. Patients with the syndrome are more likely than the general population to have genetic disorders, and about a third of people with the syndrome encountered some form of birth difficulty. In a study of 186 older patients, about ten percent had preexisting psychiatric issues. One study found that about ten percent of patients had a neurological condition before KLS developed. The condition does not appear to occur most frequently in one season.
5 (5): CD006685. doi:10.1002/14651858.CD006685.pub4. PMC 7386458. PMID 27153153. Pearce, J. M. (2008). "Kleine–Levin Syndrome: History and Brief Review". European Neurology. 60 (4): 212–4. doi:10.1159/000148694. PMID 18667831. Ramdurg, Santosh (2010). "Kleine–Levin Syndrome: Etiology, Diagnosis, and Treatment". Annals of Indian Academy of Neurology. 13 (4): 241–6. doi:10.4103/0972-2327.74185. PMC 3021925. PMID 21264130. External links

kleine_levin at NINDS
Story of young man with Kleine–Levin syndrome at The New York Times