The risk of sudden death from cardiac causes in the general population peaks from 6 a.m. to noon and has a nadir from midnight to 6 a.m. Obstructive sleep apnea is highly prevalent and associated with neurohormonal and electrophysiological abnormalities that may increase the risk of sudden death from cardiac causes, especially during sleep.

We reviewed polysomnograms and the death certificates of 112 Minnesota residents who had undergone polysomnography and had died suddenly from cardiac causes between July 1987 and July 2003. For four intervals of the day, we compared the rates of sudden death from cardiac causes among people with obstructive sleep apnea and the following: the rates among people without obstructive sleep apnea, the rates in the general population, and the expectations according to chance. For each interval, we assessed the median apnea–hypopnea index and the relative risk of sudden death from cardiac causes. We similarly analyzed sudden death from cardiac causes during three time intervals that correlate with usual sleep–wake cycles.

From midnight to 6 a.m., sudden death from cardiac causes occurred in 46 percent of people with obstructive sleep apnea, as compared with 21 percent of people without obstructive sleep apnea (P=0.01), 16 percent of the general population (P<0.001), and the 25 percent expected by chance (P<0.001). People with sudden death from cardiac causes from midnight to 6 a.m. had a significantly higher apnea–hypopnea index than those with sudden death from cardiac causes during other intervals, and the apnea–hypopnea index correlated directly with the relative risk of sudden death from cardiac causes from midnight to 6 a.m. For people with obstructive sleep apnea, the relative risk of sudden death from cardiac causes from midnight to 6 a.m. was 2.57 (95 percent confidence interval, 1.87 to 3.52). The analysis of usual sleep–wake cycles showed similar results.

People with obstructive sleep apnea have a peak in sudden death from cardiac causes during the sleeping hours, which contrasts strikingly with the nadir of sudden death from cardiac causes during this period in people without obstructive sleep apnea and in the general population.