Losing about 10 percent of their body weight was enough to bring on total or near-total remission, said Gary Foster, head of the Center for Obesity Research and Education at Temple University in Philadelphia, and lead author of the study.

"It's been clear that obesity increases the risk of sleep apnea but less clear that if obese people or people with type 2 diabetes lost weight, it would result in significant improvements in their sleep apnea -- and it did," said Foster.

People who are overweight or obese are much more likely to have obstructive sleep apnea, a condition in which a person's breathing stops or becomes very shallow, sometimes several hundred times a night and sometimes for as long as a minute, according to the American Sleep Apnea Association.

"The soft palate in the back of mouth falls down and blocks the airway," said Dr. Mitchell Roslin, chief of bariatric surgery at Lenox Hill Hospital in New York City. "When you get to people with serious levels of obesity, it's virtually impossible to find those without [this type of] sleep apnea."

The condition can lead to cardiovascular problems, including stroke, and can raise the risk for dying prematurely.

"It really has tremendous detrimental effects on the cardiovascular system," Roslin said.

The study involved 264 obese men and women who also had type 2 diabetes and obstructive sleep apnea. They were randomly assigned to an intensive behavioral program intended to encourage weight loss or to a less intensive set of group sessions that mainly addressed the issue of diabetes management.

After a year, those in the intensive program had lost an average of about 24 pounds, compared with slightly more than a one-pound average weight loss for the others.

Those who lost the weight also saw a substantial reduction in the number of sleep apnea episodes they experienced, with more than three times as many people in the intensive group experiencing complete remission (13.6 percent versus 3.5 percent).

"The greatest benefit was seen in men and those with severe apnea," Foster said.

Any amount of weight loss brought on an improvement, but those who lost about 10 percent of their original body weight saw the greatest effect. "Any weight loss is good," Foster said.

Most experts recommend 10 percent as the weight loss needed to improve sleep apnea.

However, the study also found that people whose weight remained stable experienced a worsening in their sleep apnea. Just why that occurred remains unclear.

"This is one of the first and certainly the largest study ever conducted so we're at the point in the field, unfortunately, where we're just describing the effect," Foster said.

The study, published Sept. 28 in the Archives of Internal Medicine, does seem to confirm what common sense and experience have shown.

"We've seen that when patients gain five to 10 pounds, their sleep apnea is much worse. If they lose five to 10 pounds, the sleep apnea is much better," said Dr. Hormoz Ashtyani, director of pulmonary critical care and sleep medicine at Hackensack University Medical Center in New Jersey. "It's usually not a resolution, but it's a significant improvement."