Acute myocardial infarction, sudden cardiac death and ischemic stroke show a circadian periodicity, occurring more frequently in the morning. Because arterial thrombosis is common to these disorders, a circadianicity of prothrombotic and antithrombotic processes may contribute to explain the 24 h rhythms of these acute events. Available data indeed indicate that coronary vasomotor tone, platelet aggregability, plasma inhibition of fibrinolysis, coagulant activity and blood viscosity are all greater in the morning than at other times of day, producting a relative prothrombotic state in the early morning.