It's normal for your muscles to lose tone when you sleep—that's why people in meetings nod off so amusingly. But in as many as 10 percent of middle-aged and older adults, the throat muscles relax so much that the airway repeatedly closes up, a condition that can be deadly, as it turns out. According to a study published this week in the New England Journal of Medicine, obstructive sleep apnea sharply increases the risk of stroke or death.

People with sleep apnea often don't realize they have it, since they don't remember waking up again and again, gasping for breath. Often, it's a bed partner who hears the choking and "industrial-strength snoring," says Klar Yaggi, a sleep specialist at Yale who led the study. He and his colleagues followed two groups of patients who were tested for sleep apnea (defined as stopping breathing five or more times per hour). Some had the condition; some didn't. During the 3½ years or so that they were studied, the people with sleep apnea were about twice as likely to have a stroke or die.

No one really knows why, although the explanation could have to do with the spikes of adrenaline that course through the body when breathing stops, increasing blood pressure, or with repeated plunges in the level of oxygen in the blood. This study didn't look at whether treatment—sleeping with a contraption that continuously blows air into the mouth—lowers risk. But losing weight will improve sleep apnea. And patients who use the machine get much more rest, Yaggi says, which should help them avoid one of the other major dangers of sleep apnea: car accidents.

Another study in the same issue of the New England Journal looked at how well the continuous air treatment works for people with central sleep apnea, a different disorder altogether. In both forms of sleep apnea, you stop breathing periodically. But in central sleep apnea, the problem is not an obstructed airway but that the brain fails to send out the command to breathe. The disorder is usually caused by congestive heart failure, in which the heart doesn't pump as well as it should and fluid collects in the chest. Researchers think that providing a continuous air flow during sleep might help drive water out of the lungs and make breathing more regular.

The treatment did help people with central sleep apnea in some ways: Their hearts worked better, they didn't stop breathing as often, they didn't have adrenaline surges, and they were able to exercise more.

"That's the good news," says Douglas Bradley, a pulmonologist at the University of Toronto and author of the article. "The bad news is that we didn't improve survival."

The treated patients weren't any less likely to die in the follow-up period than those who were not given the treatment. While Bradley suspects a larger study would prove a lower risk of dying, he says the benefits shown in this study aren't significant enough to recommend using the treatment routinely in people with central sleep apnea.

Information about sleep apnea from the National Institutes of Health: www.nhlbi.nih.gov

The American Sleep Apnea Association is a patient organization: www.sleepapnea.org/