WEDNESDAY, April 27 (HealthDay News) -- If you suffer a heart attack in the morning, it is likely to be more severe than having one at any other time of day, a Spanish research team reports.

In fact, if the heart attack happens between 6 a.m. and noon, it is likely to damage about 20 percent more heart muscle than a heart attack occurring in the afternoon or evening, the researchers added.

"It is well-known for several decades that the incidence of heart attack is variable across the time of the day, with higher incidence in the early morning hours," said lead researcher Dr. Borja Ibanez, from the National Center for Cardiovascular Research in Madrid.

It has been speculated, though not proven, that the body's circadian clock triggers the release of substances into the bloodstream that make the heart more prone to a heart attack at certain times of the day, he added.

"What was completely unexplored was the effect of the time of the day of onset of the heart attack on the extent of heart muscle death," Ibanez said. "This is the very first examination showing that the human heart has a variable tolerance to ischemia according to the time of the day."

The report was published in the April 27 online edition of Heart.

For the study, Ibanez's group collected data on 811 patients who had heart attacks between 2003 and 2009. Specifically, they looked at the amount of heart muscle damage in relation to the time the heart attack occurred.

The researchers found the most damage happened when the heart attack occurred between 6 a.m. and noon, compared with those whose attack occurred between 6 p.m. and midnight.

Ibanez's team determined the amount of damage done to the heart muscle by looking at the amount of two enzymes released during the attack -- creatine kinase and troponin-l. People whose heart attack occurred in the morning had 21 percent higher levels of these enzymes compared to those whose attack happened later in the day, they noted.

Among the patients studied, 269 had morning heart attacks, 240 had their attack in the afternoon, 161 had heart attacks between 6 p.m. and midnight and 141 had an attack between midnight and 6 a.m.

In addition, attacks that occurred to the back wall of the heart caused more damage than heart attacks in other locations in the heart, they found.

The findings have important implications, Ibanez said. "From a treatment point of view, when heart attacks happen in the early morning hours a more aggressive management of the case could result in better outcomes," he said.

In addition, knowing that morning heart attacks cause more heart damage might help in developing new drugs that could target the specific causes of these attacks, Ibanez said.

Dr. Gregg Fonarow, associate chief of cardiology at UCLA's David Geffen School of Medicine, said that "there is circadian variation in the incidence of heart attack, with a higher incidence in the sleep-to-wake transition period."

This circadian rhythm may include fluctuations in the nervous system, cortisol levels and other factors that up the risk of heart attack. There may also be circadian differences in how heart cells function, he said.

"However, irrespective of time of day, achieving timely reperfusion of blood with direct coronary intervention or breaking up clots is critical for patients with a myocardial infarction," Fonarow said.

More information

For more information on heart attacks, visit the American Heart Association.