Narcolepsy is a sleep disorder in which affected persons repeatedly fall asleep during the normal working hours, and are unable to keep from falling asleep. Although the cause is unknown, it tends to run in families. Thus, there may be a genetic relationship. It usually begins in healthy teens and young adults, and then continues. It is best treated by a physician who is knowledgeable about the disorder, and treatment usually involves careful use and monitoring of certain prescribed stimulant medications.