Loud snoring can be a problem, and not just for other people trying to sleep with the noise. It can also be a sign of sleep apnea. People with this condition repeatedly stop breathing while they sleep -- and may not even know it. Dr. David Gross at the National Rehabilitation Hospital of Washington says: "Sleep apnea means that the airway, the upper airway, cuts off at night ... The muscles get all relaxed and cut off and this can happen over and over again, sixty to one hundred times an hour."Most people who snore do not have sleep apnea. But doctors say most people with sleep apnea do snore. Sleep apnea not only reduces sleep quality and makes people feel tired during the day. More and more studies show that it can also lead to strokes and heart attacks. Dr. Michael Twery at the National Institutes of Health explains why: "Whenever we run out of enough air to breathe, it sends alerting signals to our minds. It raises the level of stress hormones. It tells our heart to work harder."When a person stops breathing, oxygen levels in the blood decrease. This happens again and again with sleep apnea. Dr. Twery compares the effect of sleep apnea to racing a car engine for long periods of time: "Our heart becomes overworked and we become more vulnerable to heart attack." And also strokes. Dr. Twery led a study of about nine thousand people who had sleep apnea but no history of heart disease. The nine-year study showed that the more severe the sleep apnea, the greater the chance of a stroke. Dr. Twery says it found that "men can experience up to a three-fold increased risk of stroke."Sleep apnea seems to be more common in men than in women, and it becomes more common as people get older. The most common form is called obstructive sleep apnea. People who have it are often overweight or have it in their family, but it can affect anyone. In children, for example, enlarged tonsils in the throat can interfere with breathing as they sleep. The next step in research will involve sleep apnea patients who have already had a stroke or heart attack. Researchers will study whether patients can reduce the risk of a second one with a machine called a CPAP. CPAP is continuous positive airway pressure. It provides a flow of air into the throat and lungs while the person sleeps.

For VOA Special English, I'm Carol Presutti. To read and listen to more stories and for English teaching activities, go to voaspecialenglish.com.