Circadian variation in the frequency of sudden cardiac death.

To determine whether sudden cardiac death exhibits a circadian rhythm similar to that recently demonstrated for nonfatal myocardial infarction, we analyzed the time of day of sudden cardiac death as indicated by death certificates of 2203 individuals dying out of the hospital in Massachusetts in 1983. The data reveal a prominent circadian variation of sudden cardiac death, with a low incidence during the night and an increased incidence from 7 to 11 A.M. The pattern is remarkably similar to that reported for nonfatal myocardial infarction and episodes of myocardial ischemia. The finding that the frequency of sudden cardiac death is increased in the morning is compatible with hypotheses that sudden cardiac death results from ischemia or from a primary arrhythmic event. Further study of the physiologic changes occurring in the morning may provide new information supporting or refuting these hypotheses, thereby leading to increased understanding and possible prevention of sudden cardiac death.