Heart attacks that occur in the morning are likely to be more serious than attacks at other times of the day, a specialist journal reported on Wednesday.
Spanish researchers looked at data from 811 patients who had been admitted to a Madrid clinic with a myocardial infarction between 2003 and 2009.
They used levels of an enzyme in the blood to measure the extent of dead tissue, known as an infarct, which is caused by blocked blood supply to heart muscle.
Those who had had a heart attack between 6:00 am and noon, during the transition from darkness to light, had a 21-per cent larger infarct compared with patients whose attack occurred between 6:00 pm and midnight.
The study, published in the British journal Heart, builds on previous evidence that circadian rhythm - the famous "body clock" - influences the heart in many ways such as blood pressure, heart rate, cardiac output and the function of cells that line cardiac blood vessels.
Of the 811 patients, 269 had their heart attacks in the period from 6:00 am to noon. More than three-quarters of the patients admitted to the clinic were men. Their average age was 62.
The findings should be useful in assessing the effectiveness of drugs against heart disease, the researchers said.