What you've seen in the movies is partly true -- narcoleptics do suddenly fall asleep, but most are treated with medication so that this doesn't happen.

Narcolepsy is a neurological sleep disorder first identified in 1880 by Jean-Babtiste Gelineau. Characterized by uncontrollable, recurring episodes of daytime sleep lasting from 15 minutes to an hour, this disorder afflicts approximately 135,000 Americans and has no known cure. Signs of narcolepsy usually begin to appear between 15 and 30 years of age. The four main symptoms are excessive daytime sleepiness, cataplexy, sleep paralysis and sleep hallucinations (see sidebar). Other symptoms can include troubled or fitful nighttime sleep, frequent awakenings and nightmares.

Although the exact cause of narcolepsy is currently unknown, there are many theories regarding the source of this disorder. Some of the most recent studies indicate that narcolepsy could be related to a lack of the substance hypocretin in the brain .

Scientists at the University of California in Los Angeles studied the brains of a group of narcoleptics and found that they had 85 percent to 95 percent fewer neurons containing hypocretin peptides than did non-afflicted people in a control group. Hypocretin peptides are a kind of neurotransmitter in the hypothalamus involved in regulating sleep and appetite. The scientists found that the level of melanin-concentrating hormone neurons, also found in the hypothalamus, was the same in narcoleptics as in non-afflicted people, isolating hypocretin-peptide levels as the significant varying factor.

The findings may lead to a treatment for the disorder. The researchers believe that placing new hypocretin cells in the brain might restore the functions that narcoleptic brains are lacking, thereby restoring normal sleep functioning. In another recent study, this treatment was proven to be effective in dogs; it has not yet been attempted in humans.

Other studies have found that genetics play a role in the development of narcolepsy in dogs, mice and humans. The researchers also found that narcoleptics' brains had signs of gliosis, an inflammatory process associated with neuronal degeneration, and they believe that this may be the cause of the low number of hypocretin neurons. The findings do not clearly indicate what causes the loss of these neurons, but the researchers believe it could be caused by autoimmune attacks on the neurons or certain sensitivities to environmental toxins.

On the next page, learn more about narcolepsy symptoms and treatment.