Excessive daytime sleepiness is one of the most common sleep-related patient symptoms, and it affects an estimated 20 percent of the population. Persons with excessive daytime sleepiness are at risk of motor vehicle and work-related incidents, and have poorer health than comparable adults. The most common causes of excessive daytime sleepiness are sleep deprivation, obstructive sleep apnea, and sedating medications. Other potential causes of excessive daytime sleepiness include certain medical and psychiatric conditions and sleep disorders, such as narcolepsy. Obstructive sleep apnea is a particularly significant 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 expected to increase. The evaluation and management of excessive daytime sleepiness is based on the identification and treatment of underlying conditions (particularly obstructive sleep apnea), and the appropriate use of activating medications.