The 20 million Americans who have OSA are more likely to die suddenly during those 8 hours than during the other 16 hours of the day combined.

This increased risk among patients with OSA is even more striking because it comes when cardiac deaths in the general population are at their low point.

General Population Most Vulnerable in Morning

“The cardiac death peak for the general population comes just after waking, from six to noon, which for several reasons is the most vulnerable time for the heart and blood vessels.” says Virend Somers, M.D.,PhD., the Mayo Clinic cardiologist who directed the study.

“Almost twice as many people die of cardiac causes then, as compared to the midnight to 6 a.m. period. But for patients with obstructive sleep apnea, the peaks were reversed; more than twice as many cardiac deaths came during the sleeping hours.”

Apoor Gami, M.D., lead author of the study, examined the death certificates of 112 Minnesota residents who had sleep studies at the Mayo Clinic Sleep Disorder Center between 1987 and 2003 and who died suddenly of cardiac causes. More than half (54 percent) of the 78 OSA patients died between 10 p.m. and 6 a.m., while only 24 percent of the 34 cardiac deaths among non-OSA patients occurred during that period.

Wet Paper Straw

OSA is a collapse -- like a wet paper straw -- of the airway during sleep. It causes the person to stop breathing momentarily, as many as 60 times per hour. This significantly lowers oxygen levels in the bloodstream, elevates nighttime blood pressure and causes heart rhythm disturbances. About a fifth of North American adults have sleep apnea, Dr. Gami pointed out, but most remain undiagnosed.

Snoring is often a symptom, as is inability to stay awake during the day. The standard treatment is continuous positive airway pressure (CPAP), a mask worn at night to keep the airway open.

The team’s findings not only show that patients with OSA are at much higher risk of cardiac death during sleep, but also that the 6 a.m.-noon peak of cardiac deaths in the general population is even higher for those who don’t have sleep apnea, Dr. Gami says.

“Because so many people have sleep apnea and are more likely to die suddenly during sleep, the early waking hours are even more dangerous for the rest than we had previously realized,” Dr. Gami explains. “We clearly have two distinct populations, with opposite times of highest cardiac death risk.”

Why People Die in Their Sleep

The study could not determine whether OSA raises the overall risk of sudden cardiac death or whether it simply shifts the risk to the sleeping hours, Dr. Somers pointed out.

The researchers also could not tell whether CPAP devices reduced the nocturnal death risk, because records of whether they had been used in the days before sudden death were unavailable. Previous studies have proven CPAP effective in OSA symptom relief and in raising nighttime oxygen levels in the blood, however.

“At the very least, the study may help us better understand why people should die in their sleep at all. We now know that persons with sleep apnea have a peak in sudden cardiac death risk at a time when the general population is relatively protected,” Dr. Somers concludes. “Because so many are undiagnosed, we should increase our efforts to identify them and provide the appropriate advice and treatment.”