Obstructive sleep apnea, characterized by short but frequent interruptions in breathing, may raise cardiac death risk in the small hours of the night, new research suggests.

The study of 112 Minnesota residents who died suddenly from cardiac causes found people with obstructive sleep apnea were twice as likely to die during that time than those without the condition.

That's a striking departure from the pattern of sudden death in the general population, which most often strikes in the hours right after awakening, said lead researcher Dr. Virend K. Somers, a professor of medicine and cardiology at the Mayo Clinic, in Rochester, Minn.

"We have always been curious about why cardiovascular deaths can occur in the early hours of the morning, when people are peacefully sleeping," Somers said. The study, published in the March 24 issue of the New England Journal of Medicine, suggests that for people with sleep apnea, sleep can be far from a time of quiet rest.

Sleep apnea consists of frequent stoppages of breathing lasting about 10 seconds -- not enough to shake someone awake permanently, but enough to deprive a person of rest and cause drowsiness and lethargy throughout the following day.

More than 17 percent of Americans are affected by sleep apnea, according to the researchers. Some cases are caused by problems with the brain center that controls sleep, but most are due to collapse and obstruction of the air passages.

Sleep apnea disturbs heart function in several ways, Somers said.

"If you hold your breath as long as you can, your blood oxygen levels drops to 90 percent of normal," he said. "In sleep apnea, it drops to 60 percent of normal."

"And it is not just the fact that the body is subjected to low levels of oxygen during the night," Somers added. "Sleep apnea tightens blood vessels and significantly raises blood pressure. With low levels of oxygen and high levels of carbon dioxide, several kinds of abnormal heart rhythms can develop during sleep apnea."

Unfortunately, the study does not provide any new way to manage the problem, Somers said.

"There is no clear message that is patient-oriented," he said. "It is one more brick in the wall about why we should be more aggressive in finding sleep apnea and treating it."

One simple treatment is to tell people to sleep on their side, not on their back. Another effective method is to attack the obesity that is a major cause of sleep apnea, Somers said, since weight reduction can often provide significant relief. If ordinary dieting and exercise fail, some patients may want to consider bariatric surgery, which shrinks the stomach and reduces the body's ability to take in calories, he said.

Surgery to reduce obstruction of the respiratory tract usually is not effective, Somers noted: "Surgical treatment often results in a recurrence of apnea six months to a year later."

Some people are helped by CPAP -- continuous positive airway pressure, in which air is forced into the body through a mask, he said, and "in really severe cases we can go to temporary measures such as tracheotomy," in which surgeons implant a breathing tube.

The study "is part of a growing body of evidence that this disease called sleep apnea is deadly," said Dr. Steven H. Feinsilver, a member of the board of directors of the American Sleep Apnea Association.

While the snorts and snoring caused by sleep apnea are unmistakable to anyone in the same room, people who sleep alone often don't realize they have the condition, Feinsilver added.

"Most people with sleep apnea know only that they are sleepy during the day," he said. Treatment is necessary "not only because it improves sleep, but also because treatment helps people live longer," he said. "It is a major risk factor for heart disease and stroke, and we are just beginning to learn how dangerous it is."