DEAR MAYO CLINIC: Can you explain narcolepsy to me - how it develops, the treatment options, and what someone with the condition goes through? A friend of mine was recently diagnosed, and I don't know anything about it but would like to learn more.

ANSWER: Narcolepsy is a neurological condition that makes people sleepy all the time. Those who have the condition fall asleep frequently during the day, and they may not sleep well at night. Narcolepsy is thought to be caused by low levels of hypocretin, an important brain chemical that helps control when a person is awake.

Narcolepsy is diagnosed using sleep studies. Although there's no cure, the disorder usually can be effectively treated with medication.

The normal process of falling asleep begins with non-rapid eye movement sleep (NREM). During this phase, brain waves slow down. After an hour or two of NREM sleep, brain activity picks up again, and rapid eye movement sleep (REM) begins. Most dreaming occurs during REM sleep.

In people who have narcolepsy, the normal sleep process is disrupted. They enter into REM sleep more rapidly and may experience intrusions of REM sleep into wakefulness. As a result, phenomena that normally occur during REM sleep - such as muscle paralysis and vivid dreams - may occur when people with narcolepsy are awake.

Most people with narcolepsy have very low levels of hypocretin, although what causes these low levels isn't entirely clear. Some research suggests it may be linked to an autoimmune disorder in which a person's immune system mistakenly attacks the cells that produce hypocretin. Narcolepsy also appears to have a genetic component. People who have relatives with narcolepsy are at increased risk of developing the condition. Research is under way to study further the underlying cause.

Narcolepsy can have a devastating effect on daily life. Excessive sleepiness makes it hard to function and focus on tasks, and can result in poor performance at work or school. The prospect of suddenly falling asleep makes driving dangerous. In addition, narcolepsy is often accompanied by cataplexy, a condition that causes a sudden loss of muscle tone. Triggered by strong emotion, cataplexy can result in symptoms such as slurred speech, a drooping jaw or slumping shoulders. In severe cases, cataplexy can cause a person's knees to buckle, resulting in a collapse.

Other sleep problems are associated with narcolepsy. Hallucinations just before falling asleep or immediately after waking (hypnagogic or hypnopompic hallucinations respectively) are common. REM sleep behavior disorder - a condition in which people act out their dreams by flailing their arms or kicking and screaming - is seen more frequently in people with narcolepsy. Also, they often experience a temporary inability to move or speak while falling asleep or upon waking (sleep paralysis).

Diagnosis of narcolepsy is based on a person's symptoms combined with the results of sleep studies. Polysomnography is used to assist in diagnosis of the condition. Using electrodes placed on the scalp, polysomnography measures a variety of signals during sleep, including the electrical activity of a person's brain and heart, muscle movement, eye movement and breathing. A daytime polysomnogram called a multiple sleep latency test is required for a definitive diagnosis of narcolepsy. This sleep study provides an objective measure of sleepiness.

Once diagnosed, narcolepsy can often be successfully managed. Standard treatment involves drugs that stimulate the central nervous system and help people with narcolepsy stay awake during the day. Antidepressant medications, which suppress REM sleep, are often used to help alleviate the symptoms of cataplexy. Sodium oxybate (Xyrem) can control cataplexy and helps improve nighttime sleep. It may also help control daytime sleepiness.

In addition to medication, lifestyle changes such as following a consistent sleep and wake schedule and taking short naps at regular intervals may help manage some narcolepsy symptoms.

(Medical Edge from Mayo Clinic is an educational resource and doesn't replace regular medical care. E-mail a question to medicaledge@mayo.edu , or write: Medical Edge from Mayo Clinic, c/o TMS, 2225 Kenmore Ave., Suite 114, Buffalo, N.Y., 14207. For more information, visit www.mayoclinic.org.)