Researchers have found that the greatest amount of injury to the heart occurs when individuals have a heart attack between the times of 1:00 am and 5:00 am.

Previous animal studies of circadian rhythm link to cardiovascular injury found that the heart attack size in rodents following reduced blood supply and blood supply return displayed a dependence on the time of heart blockage.

"We were trying to ascertain whether the time of day of when a heart attack occurs influences the amount of damage that the heart sustains, or was this just a phenomenon exhibited in rodents," said the study's senior author Jay H. Traverse, MD, a cardiologist at Abbott Northwestern Hospital in Minneapolis.

The study was published in the journal Circulation Research.

Reviewing Heart Attack Data

Researchers analyzed data of around 1,000 patients in the level 1 acute MI database with an acute heart attack. Breaking down the reduced blood supply time between one and six hours researchers were able to identify 165 patients with their first heart attack, who had blocked arteries with evidence pre heart attack pain (pre-infarction angina) or collateral blood flow. All of the 165 patients had marked reduced blood flow times and the data supported with cardiac MRI measurement of infarct size, or size of heart attack, and area at risk.

Impact of Time

The researchers observed that the extent of heart attack size was significantly associated with the time of day. The greatest Heart (myocardial) injury occurred at 1:00 am onset of reduced blood supply and 5:00 am onset of reperfusion (injury which takes place after blood supply returns to tissue) with the peak injury being 82 percent higher than that recorded at lowest time of injury.

"It is important to understand that the heart's ability to protect itself against more severe damage varies over a 24-hour cycle. Identifying those protective changes may be particularly relevant for pharmaceutical manufacturers that are seeking to develop cardio protective drugs," said Dr. Traverse