The common form of sleep apnea, in which the throat closes off throughout the night, at least doubles the risk of stroke or death, a study released on Wednesday showed.

The researchers at Yale University also raised questions about whether existing apnea treatments reduced that risk, the study published in The New England Journal of Medicine showed.

A separate Canadian study also published in the journal concluded that breathing machines used to treat a form of apnea common in people with heart failure do not prevent death or the need for a heart transplant.

About 4 percent of Americans suffer from extreme daytime fatigue because their sleep is disrupted by sleep apnea, forcing them to repeatedly wake up if only for a second. Another 16 percent of Americans have apnea without fatigue.