Narcolepsy is a chronic sleep disorder that causes excessive, overwhelming daytime sleepiness.

What is going on in the body?

A person with narcolepsy has a lifelong sleep disorder. The central nervous system tells the body when to sleep and when to wake. In a person with narcolepsy, these messages are confused. The messages to sleep and wake happen at the wrong times. The body falls asleep when the person wants to be awake. The body can also be awake when the person wants to be sleeping.

What are the causes and risks of the condition?

A recent study has shown that individuals with narcolepsy are missing cells from the hypothalamus that secrete a hormone called hypocretin, or orexin. On autopsy, the brains of people with narcolepsy showed clear evidence that the cells had been destroyed, perhaps by an autoimmune disorder or a toxin. An autoimmune disorder is one in which the person's body attacks its own tissues, for unknown reasons.

Since narcolepsy has been shown to run in families, there may be a genetic component to the condition. About 1 in 2,000 people has narcolepsy, and most of these have their first symptoms between the ages of 15 and 30.