Recent evidence suggests that acute myocardial infarctions tend to occur most frequently in the early morning hours after awakening. In addition, the peak time of occurrence of out-of-hospital sudden death is during the same morning period, which suggests that the pathophysiologic factors influencing these acute ischemic and arrhythmic events may be similar. Although it is clear that arising from sleep is associated with an increased heart rate and blood pressure, two important determinants of myocardial oxygen demands, other factors may be responsible for a paradoxical decrease of coronary blood flow at the same time. It is possible that vascular receptor sensitivity may vary or that the early morning hours may be associated with a hyperaggregability of platelets; this would provide an additional pathophysiologic explanation for ischemic events related to platelet aggregation and coronary thrombosis, which are known to play a central role in the initiation of unstable angina, acute infarction, and sudden death. An understanding of the circadian pattern of ischemic events may be a guide for future therapeutic interventions during this period. Studies suggest that use of medications that provide continuous antiischemic activity throughout the 24-h day should be considered.