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This chapter presents a summary of excessive sleepiness definitions used in epidemiological studies. Studying prevalence, incidence and risk factors for excessive daytime sleepiness bears little impact on the development of new treatments for this symptom. Three sleep disorders are characterized by excessive sleepiness and are divided into 12 diagnoses: hypersomnia, behaviorally induced insufficient sleep syndrome, and narcolepsy. Hypersomnia and behaviorally induced insufficient sleep syndrome are virtually undocumented in the general population. Excessive sleepiness can be caused by various factors such as poor sleep hygiene, work conditions, and psychotropic medication use. Excessive sleepiness has been found to be associated also with sleep-disordered breathing, psychiatric disorders, especially depression, and physical illnesses. Excessive sleep quantity is an associated symptom in depressive disorders in the DSM-IV classification. Several clinical studies have also pointed out the high occurrence of subjective excessive sleepiness in association with mental disorders, organic disorders, or both.