The onset of acute myocardial infarction (AMI) is a complex interplay of internal circadian factors and external physical and emotional triggers. These interactions may lead to rupture of an often nonocclusive vulnerable atherosclerotic coronary plaque with subsequent formation of an occlusive thrombus. The onset of AMI has a distinct pattern, with peak incidence within the first few hours after awakening, on certain days of the week, and in the winter months. Physical and emotional stresses are important triggers of acute cardiovascular events including AMI. Triggering events, internal changes, and external factors vary among different geographical, environmental, and ethnic regions. Life-style changes, pharmacotherapy, and psychologic interventions may potentially modify the response to, and protect against the effects of triggering events.