Excessive daytime drowsiness is one of the most frequent sleep-related patient symptoms, affecting an estimated 20 percent of the population. Individuals with excessive daytime sleepiness are at risk of motor vehicle and work-related mishaps and have poorer health than comparable people. The most prevalent causes of excessive daytime sleepiness include sleep loss, obstructive sleep apnea, and sedating drugs. Other probable reasons for excessive daytime sleepiness include medical and psychological diseases and sleep disorders, such as narcolepsy. Obstructive sleep apnea is a particularly prominent cause of excessive daytime sleepiness. An estimated 26 to 32 percent of adults are at risk of or have obstructive sleep apnea, and the prevalence is predicted to increase. The diagnosis and therapy of excessive daytime drowsiness are focused on detecting and treating underlying disorders (especially obstructive sleep apnea) and the appropriate use of activating drugs.