Q. For the past few months, I've felt tired a lot. I find myself falling asleep several times a day, even though I've been getting more sleep than usual, sometimes 10 or more hours. At night, I sleep pretty well, although I sometimes wake up during really vivid dreams. What worries me is that I've dozed off a few times while driving. This has happened often enough that I'm afraid to drive myself anywhere. I've also nodded off in the middle of a conversation with friends. What could be making me so tired? Is there anything I could take?

A. I think you have narcolepsy, a condition that makes you feel tired and fall asleep easily, sometimes suddenly and without warning. Although there's no cure, several medicines can help.

Narcolepsy afflicts about one of every 4,000 people. Many people with the condition are unaware that they have it, attributing the symptoms to stress, fatigue or insufficient sleep. It tends to run in families, so if you're diagnosed with narcolepsy you should urge other family members to be checked, especially if they have also displayed symptoms. Also, you should not drive until your symptoms are under control.

The first signs of narcolepsy usually start appearing during the teenage years, although they can appear anywhere between ages 5 and 50. In addition to sleepiness and episodes of falling asleep, narcolepsy symptoms may include cataplexy, hypnagogic hallucination and sleep paralysis.

Attacks of cataplexy involve a loss of muscle control that will cause you to feel very weak. Your legs may buckle, and you may even fall to the ground. These attacks are usually triggered by some strong emotion, such as laughter or anger.

Hypnagogic hallucinations are vivid dreams that people with narcolepsy may experience when falling asleep or waking up. Your description of waking up during vivid dreams sounds like you're having these.

Sleep paralysis would cause you to feel partially or completely paralyzed for a brief time before falling asleep or after waking up.

Most people with narcolepsy have attacks of cataplexy, hypnagogic hallucination or sleep paralysis from time to time.

Doctors can generally tell if you have narcolepsy from the symptoms you report. But to be sure, you'll need to have a sleep study done. You'll go to a sleep lab, either at a hospital or sleep center. There, you'll be hooked up to some monitoring devices. Doctors will study your sleep patterns and check for telltale signs of narcolepsy on repeated brain wave recordings. Your doctor will also check to make sure your symptoms aren't being caused by some other condition, such as sleep apnea.

For treatment, your doctor may prescribe stimulant medication to keep you from falling asleep so easily. The one most often used is methylphenidate, also know as Ritalin. You may also be prescribed other stimulants such as pemoline or amphetamines to battle sleepiness. Antidepressants may be prescribed to treat cataplexy, hypnagogic hallucinations and sleep paralysis.

For more information about narcolepsy, contact the Narcolepsy Network, P.O. Box 42460, Cincinnati, OH 45242 (telephone: 513-891-3522; Web site: www.websciences.org/narnet) or the National Sleep Foundation, 1522 K St. NW, Suite 510, Washington, DC 20005 (www.sleepfoundation.org). You can also visit the Sleepnet Web site, a site linking sleep-related Web pages: www.sleepnet.com.

Jay Siwek, chairman of the department of family medicine at Georgetown University Medical Center, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.