Kleine-Levin syndrome (KLS) is a very rare disease. It occurs most often during adolescence, and affects men more than women. The syndrome is characterised by recurring episodes of daytime sleepiness that can last from a few days to several weeks. Between these episodes, sleep duration returns to normal and daytime sleepiness disappears. KLS is also characterised by cognitive, psychiatric and behavioural disorders. In most cases, subjects experience other symptoms such as apathy, confusion and derealisation (altered perception of reality). In other, less frequent cases, subjects can suffer from eating disorders, hypersexuality, depression, anxiety and hallucinations. In the long term, episodes of hypersomnia tend to become less frequent and sometimes disappear with age.