Excessive sleepiness (hypersomnolence) is a relatively common occurrence following sleep deprivation and, similarly, a prevalent symptom accompanying a number of neurological, psychiatric and other medical disorders, infections and traumas. The presence of excessive sleepiness in neurodegenerative diseases, vascular cerebral accidents, as well as CNS injuries, tumours and infections, and the existence of central disorders of hypersomnolence represent a significant clinical challenge in neurology. Increased prevalence of hypersomnolence has also been observed in mental disorders. Considering the impact of hypersomnolence on social functioning and the quality of life, excessive sleepiness is currently subject to intense clinical research.