Fully half of the 400 women given overnight sleep tests in a new Swedish study turned out to have mild-to-severe sleep apnea.

In the random population sample of adult women who answered a questionnaire and were monitored while sleeping, half experienced at least five episodes an hour when they stopped breathing for longer than 10 seconds, the minimum definition of sleep apnea.

Among women with hypertension or who were obese - two risk factors for sleep apnea - the numbers were even higher, reaching 80 to 84 percent of women.

Many of the women in the study represent mild cases of sleep apnea.

"How important is the mild sleep apnea, we don't know," said Dr. Karl Franklin, the lead author of the study and a professor at Umea University in Sweden.

Terry Young, a professor in the School of Medicine and Public Health at the University of Wisconsin, said mild sleep apnea is important to pay attention to.

"We see that it doesn't go away and it gets worse," she said.

Sleep apnea is tied to a higher risk of stroke, heart attack and early death.

One recent study also found that women who have sleep apnea are more likely to develop memory problems and dementia.

Franklin said his group wanted to get updated evidence of how common the condition is.

The researchers selected 400 women between the ages of 20 and 70 from a larger population sample of 10,000, and asked them to sleep overnight at home with sensors attached to their bodies.

The sensors measured heart rate, eye and leg movements, blood oxygen levels, air flow and brain waves.

Each apnea event was defined by at a least a 10-second pause in breathing accompanied by a drop in blood oxygen levels.

Women who had an average of five or more of these events during each hour of sleep were considered to have sleep apnea.

The study, which was funded by the Swedish Heart Lung Foundation, found that apnea became more common in the older age groups.

Among women aged 20-44, one quarter had sleep apnea, compared to 56 percent of women aged 45-54 and 75 percent of women aged 55-70.

Young said these numbers are higher than her own estimate, but that's likely because she used a more strict definition of sleep apnea than Franklin's group.

Franklin also said his equipment, being newer, is more sensitive in detecting interruptions in breathing.

Severe sleep apnea, which involves more than 30 breathing disruptions per hour, was far less common.

Just 4.6 percent of women 45-54 and 14 percent of women 55-70 had severe cases.

Among women of all ages with hypertension, 14 percent had severe sleep apnea, and among women who were obese, 19 percent had severe apnea.

Franklin said that if physicians are looking for sleep apnea among women, examining those who are obese, over 55 or have hypertension is a good place to start.

Young said sleep apnea is often thought of as a condition of men, but identifying women with it is especially beneficial, because her research has shown that women are good at sticking with treatment.

"The prejudice of excluding women (as potentially having sleep apnea) has been rampant for a long time. It's gotten better, however, and the (public health) gain in identifying women with sleep apnea is great," she said.