Dr Aida Suarez-Barrientos and colleagues from the Clinical Hospital San Carlos in Madrid looked at the relationship between the time of day at which MIs occurred and the final size of the infarct in heart tissue.
The researchers retrospectively analysed the effect of the time-of-day onset of acute MI on final infarct size. They studied 811 patients who presented with a first ST segment elevation MI (STEMI).
Dr Suarez-Barrientos and colleagues found that the size of infarct varied significantly depending on when the MI occurred. In particular, early morning onset of acute MI was associated with larger MIs.
Writing online in the journal Heart, the researchers said: 'Patients with STEMI onset in the dark-to-light transition period (6am to noon) had significantly larger MIs than those with symptom onset at any other time of the day.
'Overall, there is an expected increase of about 20 per cent in infarct size in patients with STEMI onset in the dark-to-light transition period compared with any other time of day.
They also found a significantly higher incidence of acute MI and anterior wall acute MI in the early morning hours, and anterior wall acute MI was also significantly associated with larger infarcts.
The researchers believe their study is the first report of the impact of time of day on STEMI onset on infarct size.
They suggest that 'time of acute MI onset' should be considered when assessing the effectiveness of new therapies designed to reduce cardiovascular risk.