A circadian rhythm is an endogenously driven, roughly 24-hour cycle in biochemical, physiological or behavioral processes.  Previous research in rodents has shown that the infarct size following ischemia and reperfusion exhibits a circadian dependence on the time of coronary occlusion. Jay H. Traverse, from the Minneapolis Heart Institute (Abbott Northwestern Hospital; Minnesota, USA), and colleagues completed a retrospective analysis of 1,031 patients in the Level 1 acute MI database with an acute heart attack, or ST-elevation myocardial infarction (STEMI), referred for primary percutaneous coronary intervention (PCI) with known ischemic times between one and six hours.  The researchers observed that the extent of infarct size was significantly associated with time of day onset of infarction. The greatest myocardial injury occurred at a 1:00am onset of ischemia and 5:00am onset of reperfusion with the peak injury being 82% higher than that recorded at lowest time of injury.  The study authors submit that: “The results of this study demonstrate for the first time in humans that myocardial infarct size and left ventricular function after STEMI have a circadian dependence on the time of day onset of ischemia.”