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People with moderate to severe sleep apnea may also be at greater risk of having a stroke, a new study suggests.

A team of Canadian and U.S. researchers based their findings on data from 1,475 participants who were tested for “sleep-disordered breathing” at the University of Wisconsin. Of the group, about 7 per cent had moderate to severe sleep apnea.

Follow up visits were conducted over the next four, eight and 12 years. The results suggested a “very significant” link between severe forms of the illness and stroke, independent of other factors like diabetes and high blood pressure, according to the findings.

“We found that for people with moderate to severe sleep apnea, the odds of having a stroke in the next four years were almost four times greater than if you did not have sleep apnea,” said Dr. Douglas Bradley, a senior investigator at the Toronto Rehabilitation Institute, who worked on the study.

Sleep apnea causes a person to stop breathing repeatedly during sleep because their throat collapses. The disorder is commonly linked to factors like weight gain and aging.

Although the study suggested a connection between severe sleep apnea and stroke, it found no link in those with milder forms of the disorder. Subjects were considered to have moderate to severe sleep apnea if they stopped breathing 20 times or more an hour during sleep.

The study also suggested the link may be a result of interruptions in nighttime breathing depriving the brain of oxygen and activating the sympathetic nervous system, causing increases in blood pressure and heart rate.

The most common sign of sleep apnea is loud, habitual snoring. Others include restless sleep, waking with a headache, excessive sleepiness at work or in other situations.

The findings were published in the December issue of the American Journal of Respiratory and Critical Care Medicine.