FRIDAY, May 10 (HealthDay News) -- The widening American waistline may be feeding an epidemic of sleep apnea, potentially robbing millions of people of a good night's rest, a new study suggests.

The research didn't definitively link the rise in obesity to sleep apnea, and it only looked at 1,520 people, almost all white, in Wisconsin. But study author Paul Peppard believes the findings show a big spike in sleep apnea cases over the past two decades -- as much as 55 percent -- and may translate to the entire United States.

"There are probably 4 million to 5 million people who are more likely to have sleep apnea due to the obesity epidemic," estimated Peppard, an assistant professor of population health sciences at the University of Wisconsin-Madison. "It's certainly an uncalculated cost of the obesity epidemic, an epidemic of its own."

The researchers looked at adults aged 30 to 70 who were monitored as they slept. About 600 to 700 underwent sleep tests between 1988 and 1994, with some continuing to take part along with hundreds of new participants from 2007 to 2010.

The study considered the participants to have moderate-to-severe breathing problems if they had trouble breathing 15 or more times an hour while sleeping.

Sleep apnea is the main cause of breathing problems during sleep. People with the condition often have trouble staying in deep sleep because their throats close, blocking their airways and requiring them to partially awaken to start breathing properly. They don't realize they're waking up and may become very sleepy during the day.

Besides sleepiness, sleep apnea can contribute to heart and other health problems if untreated and increase the risk of work- and driving-related accidents, according to the U.S. National Heart, Lung, and Blood Institute.

The researchers extrapolated their findings to the entire United States and estimated that 10 percent of men aged 30 to 49 currently have symptoms of sleep apnea. The study estimates the number is 17 percent of men aged 50 to 70. For women, the estimate is 3 percent among those aged 30 to 49 and 9 percent among women aged 50 to 70.