Sleeping used to be one of my favorite activities -- until I got lousy at it. I started having trouble with it last year when I found myself feeling more tired when I woke up than when I went to sleep. I practically fell asleep at my desk. (Okay, I DID fall asleep on my keyboard.) My once respectable memory took a precipitous decline into the 'I can't remember my own telephone number' range. And then there was the problem, for my husband and anyone within a three-room radius, of the snoring. So I asked some doctors to tell me why I was so exhausted. One said I was super-anxious and should get massages. (I didn't feel anxious.) Another said I was depressed. (I wasn't.) A third said that it could be hormonal. (What isn't?) Or not. Finally I decided to figure it out myself. After investigating possible causes for all my symptoms, I began to suspect I had apnea. Sleep apnea, it turns out, is a common disorder in which you momentarily stop breathing, or take very short breaths, while you are sleeping. The number of times your breathing is interrupted per hour determines the degree of your apnea. (A mild case is marked by five to 15 episodes; a severe case involves more than 30.) These interruptions disturb your sleep patterns, possibly reducing the body's ability to refresh itself and the brain's ability to consolidate memories. The interruptions also mean that your brain doesn't get all the oxygen it needs to function well, and they can cause high blood pressure, hypertension and stroke. Researchers at the National Center on Sleep Disorders Research say that tens of thousands of cardiovascular deaths annually are in some way related to sleep apnea. In 2004, apnea was linked to the death of National Football League star Reggie White at age 43. And here's more disturbing news: The National Institutes of Health estimates that at least 18 million Americans suffer from sleep apnea -- and that as many as 90 percent of them are not aware that they have the condition. One reason is that there is no easy way to tell if you have it. To see if my amateur diagnosis might be correct, I called my brother-in-law, Mark Dettelbach, who, lucky for me, is an ear, nose and throat specialist. He said that as a somewhat trim, 53-year-old woman, I wasn't the most obvious of candidates (apparently, obese men older than 50 are the most common sufferers) but that apnea was certainly possible. 'A lot of people have apnea who don't fit the stereotype,' he said. 'It can be tricky. A lot of times doctors don't think about sleep apnea in someone who does not look like a picture of the stereotypical sleep apnea patient.' Indeed, he said, thin people can get apnea (although being overweight is a risk factor). Kids get it, too, the most common cause being enlarged adenoids and/or tonsils. And even though loud snoring is one common symptom, non-snorers can have the condition. There are two basic types of sleep apnea. Obstructive sleep apnea, the more common, occurs when throat muscles relax and block passage of air; central sleep apnea occurs when the brain fails to send signals to the muscles to relax for air to flow. Some unlucky people have both. Mark instructed me to have an overnight sleep study, called a poly-somnogram, at the Sleep Disorders Clinic in Chevy Chase, Md. The clinic is run by Helene A. Emsellem, a doctor who is an expert in sleep disorders and the author of a book about teen-agers and sleep, 'Snooze ... or Lose!' 'We are seeing more and more patients presenting with fatigue that remains unexplained,' Emsellem said. 'And we are seeing them after some have been to an internist, a psychiatrist to rule out depression, a rheumatologist to rule out aches and pains,' she said. I went to the clinic one Sunday night, and a technician wired me up to electrodes to monitor brain waves, eye movements, heart rate and other functions while I slept. I was also fitted for a mask through which air would be forced -- in a technique called continuous positive airway pressure, or CPAP -- if it was determined in the first part of the sleep study that I did have apnea. It may seem difficult to sleep when you are connected to wires, but not if you are as tired as I was. I found myself being awakened at about 2 a.m. when the CPAP mask was connected to a small machine that forced air through a tube and into my nose. I indeed did have apnea. (I learned later that my breathing either stopped or was shortened 27 times an hour.) Lying there with the mask covering my nose, I felt a brief surge of anger at the doctors who hadn't diagnosed it earlier, and at the fact that I would now have to be hooked up to a machine while I slept. Then I took a deep breath of the air and felt an unusual flash of clarity. 'This isn't the worst thing in the world,' I thought. The clinic sent me home at 6:30 the next morning (after the technician looked at my results and said, 'I don't know how you get up in the morning'), and within a few days my own personal CPAP machine was delivered to my house. I didn't even have to call for it; the sleep center arranged it, and my insurance paid for it. Small enough to sit on my nightstand, it makes a low humming noise (white noise). Now I sleep better and don't snore. Still, adjusting the mask and learning to sleep in a position that keeps it on my face is something of a trick, and Emsellem said research is under way to find a less cumbersome, equally effective treatment. Doctors also say that lifestyle changes can sometimes help relieve apnea symptoms: losing weight, avoiding alcohol and quitting smoking. And surgical procedures can help alleviate some cases of sleep apnea, though they are not always successful. One such procedure, a uvulopalatopharyngoplasty (where do they get these names?), removes excess throat tissue to allow air to flow more freely; other procedures involve restructuring the jaw and other areas. For kids, the most common surgery is removing the adenoids and/or tonsils. Now I am the most annoying of converts, forever proselytizing. I quiz friends to see if they have the symptoms of sleep apnea and then urge them to undergo a sleep study. One did and learned she stops breathing while sleeping 45 times an hour. So here's the pitch: If you have any of these symptoms and can't figure out another cause, find out if you have sleep apnea. Don't wait for your doctor to figure it out for you. 