Heart attacks are more dangerous in the mornings than at any other time of the day, finds a new study from Spain.

The study found that heart attacks that occur between 6 a.m. and noon are likely to damage about 20 per cent more of the heart muscle than an attack in the afternoon or evening.

Doctors have long noticed that there are more heart attacks in the morning, though no one really knows know. Some suspect that the body's circadian clock triggers the release of hormones substances into the bloodstream that make the heart more prone to a heart attack.

But what wasn't known was how damaging these attacks can be.

So researchers at Spain's National Centre for Cardiovascular Research s reviewed data from more than 800 heart attack patients treated at a hospital cardiac centre in Madrid, between 2003 and 2009.

Among the patients, 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. All the patients had a type of heart attack caused when the blood supply is blocked for a long time, known as STEMI (ST segment elevation myocardial infarction).

The researchers looked at the amount of heart muscle damage in each patient by examining concentrations of creatine kinase (CK) and troponin-I (TnI) -- two key enzymes released in response to muscle injury.

They found that heart attacks that occurred in the morning were associated with about 20 per cent more dead heart tissue.

The report was published in the journal Heart.

The authors say that knowing that morning heart attacks can be more severe could have important implications for their treatment. For example, most cardiac catheterization labs are not fully staffed in the early morning hours, but perhaps they should be

As well, understanding why heart attacks are more common in the morning could help in developing new drugs that could target the specific causes of the attacks.