Kleine–Levin syndrome is a rare disease (1–2 cases per million inhabitants) characterised by recurrent episodes of hypersomnia with accompanying hyperphagia, hypersexuality and behavioural abnormalities such as aggression, irritability and disorientation. The diagnosis is based on clinical criteria, after ruling out other causes of hypersomnia. There are no lesions in brain imaging studies while EEG demonstrates slowing of bioelectrical activity in some cases. Kleine–Levin syndrome has a self-limiting course with symptoms becoming milder over time until they disappear completely. The pathogenesis includes hypothalamic pathology or autoimmune background (association with certain human leukocyte antigens). This article presents a young patient with a typical clinical picture of Kleine–Levin syndrome. Due to its rare occurrence, the knowledge about this disease entity is not common – the correct diagnosis was made a year after the onset of first symptoms.