A circadian variation of sudden cardiac death has been documented, but its relation to individual time of awakening and possible triggering events has not been studied in the general population. By monitoring of mortality records in 4 cities and towns in Massachusetts, 148 potential cases of sudden cardiac death were identified. In 94 cases, the informants listed on the death certificates were contacted, the diagnosis of sudden cardiac death was established, and a telephone interview was completed within a mean of 19 days (range 8 to 28) after the death. The time of day of all 94 cases of sudden cardiac death (mean age 61 +/- 9 years, 74% men) demonstrated a circadian variation (p less than 0.05) with a peak from 9:00 A.M. to 12:00 noon. An analysis of time of death adjusted for individual wake-times of the decedents demonstrated an increased onset of sudden cardiac death during the initial 3-hour interval after awakening with a relative risk of 2.6 (95% confidence interval 1.6, 4.2) compared with other times of the day. The increased risk of sudden cardiac death soon after awakening suggests specific triggering factors or mechanisms that are particularly likely to occur during this time. The narrowing of the time interval during which the risk of sudden cardiac death is increased should facilitate the study of possible pathogenetic mechanisms and triggering factors of the disease and may aid in the design of more effective preventive strategies.