Excessive daytime sleepiness (EDS), hypersomnolence, or "pathological sleepiness" is a complaint of increased sleep propensity occurring at inappropriate times that adversely affects vigilance, performance, and daytime function. A detailed history and physical examination are of key importance in evaluating the patient complaining of EDS. Insufficient sleep is the most common cause of EDS in Western culture. Narcolepsy is the most clearly understood of the central hypersomnia syndromes. Idiopathic hypersomnia is a primary disorder of EDS that has historically been given as a diagnosis of exclusion to individuals who complain of EDS when other disorders that cause hypersomnolence have not been found or clearly characterized. The Kleine-Levin syndrome is a form of recurrent hypersomnia that occurs primarily in adolescents, with a 4:1 male predominance. Sleepiness while driving accounts for a significant proportion of traffic fatalities and injuries and is recognized as a legitimate public health concern.