The 20 million Americans who have obstructive sleep apnea (OSA) are more likely to die suddenly of cardiac causes between 10 p.m. and 6 a.m. than during the other 16 hours of the day combined, according to findings of a Mayo Clinic study published this week in the New England Journal of Medicine.

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. Among the general population, just after waking, from 6 a.m. to noon, is the most common time for heart attacks to occur.

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 of the 78 OSA patients died between 10 p.m. and 6 a.m., while only 24% of the 34 cardiac deaths among non-OSA patients occurred during that period.

The reason for this difference, explained researchers, is that when a person with OSA continually stops breathing during the night, it elevates nighttime blood pressure and causes heart rhythm disturbances.

Researchers cautioned that 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. 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,” said one researcher. “Because so many [OSA cases] are undiagnosed, we should increase our efforts to identify them and provide the appropriate advice and treatment.”