Sleepiness, or hypersomnia, is a relatively common complaint and one of the main problems of modern society. Accurate evaluation and diagnosis of sleepiness are important. The methods used for evaluating sleepiness are subjective measures or self-evaluations, performance decrease measures, sleep propensity measures, and arousal decrease measures. A clear and detailed history is important in differential diagnosis of sleepiness because symptoms of sleepiness may be expressed in terms of tiredness or fatigue that do not directly denote sleepiness. Comprehensive diagnostic evaluation is also invaluable because these symptoms may result from a variety of causes ranging from medical disorders to insufficient nocturnal sleep.