Circadian variations in myocardial ischemia. Implications for management. Extended ambulatory electrocardiographic monitoring in the patient's customary environment provides clear evidence of circadian patterns in myocardial ischemic episodes. In patients with effort angina, the highest activity occurs between 6 AM and noon. This coincides with peaks in diurnal variation of frequency of acute myocardial infarction, stroke, and sudden death. A number of potential underlying common triggering mechanisms, including catecholamine secretion, sympathetic nervous system activity, blood pressure, heart rate, cortisol secretion, and aggregability of platelets, exhibit similar surges. As a result of these coinciding morning peaks, myocardial oxygen demand is increased and oxygen supply reduced after a person arises in the morning. Attention to this vulnerable period is merited in the timing and choice of medication, both to prevent or reduce ischemia and to modify potential disease-triggering mechanisms. 