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It has been dubbed the Merry Christmas coronary and the Happy New Year heart attack. More people have heart attacks in the early winter than any other time of year, and not just in cold, snowy countries like Canada.

From Hawaii to the Arctic, researchers have found a higher rate of heart attacks in winter, especially in January. One study in the United States concluded there are twice as many heart attacks a day in January as in July. A French study found a 13-per-cent increase in the risk of heart attacks with every 10-degree drop in temperature.

Winter heart attacks are also more likely to be deadly, and cause more damage to cardiac muscles than those that occur in other seasons. A study in Britain found that people who had heart attacks in the winter were 19 per cent less likely to survive.

Scientists say they can't offer a definitive explanation for the increase in the frequency and severity of heart attacks in winter, but they've identified a number of factors for why 'tis the worst season for heart trouble.

The weather is an obvious culprit, because in the cold, blood vessels constrict, clamping down to help the body save heat. This increases blood pressure, which puts more strain on the heart. Clotting factors in the blood also increase.

Then there is all that snow — shovelling is an intense workout, especially for people who don't get regular exercise.

It seems logical, then, that people who live in snowier, colder parts of the country would be most at risk. But they aren't.

Philip Jong, a cardiologist and research fellow at the Institute of Medical Sciences at the University of Toronto, says that the increase in heart problems in the winter months is actually greater in Southern Ontario than in Northern Ontario and the Northwest Territories.

“How people respond to the changing temperatures is important. In northern parts of Canada, where the temperature is always cold all the time, people are well-prepared for the cold climate. They tend to stay indoors, limit exposure and dress themselves appropriately,” says Dr. Jong, who is also a spokesman for the Heart and Stroke Foundation.

In southern parts of Canada, people may be less prepared for the cold when it strikes, he says. When a sudden cold spell hits, they go outside underdressed, and perhaps exercise strenuously.

Even the mild winter weather in places like Los Angeles may be a factor, says Dr. Robert Kloner, the L.A. doctor who is investigating why there are more heart attacks in the winter, particularly early January.

The comforts, excesses and stress of the holiday season may play a role.

“If you look at cardiac death rates, they go up at Thanksgiving, which is late November here, and peak at the beginning of the New Year,” Dr. Kloner says.

People often increase their intake of fat, alcohol and salt during the holidays. All are bad for the heart, he says. The holidays can also bring psychological and emotional stress.

“It is dealing with all those obnoxious relatives. We know that people are going to be stressed out,” he adds.

Pneumonia and influenza may also be factors. One study found that the risk of heart attack can triple in the 10 days after a serious respiratory infection.

The cheerful Yule log may also play a role. Burning wood in a fireplace or wood stove can release particulates that are damaging to the heart and lungs.

U.S. researchers previously wondered if heart attacks were deadlier in the early winter because hospitals were short-staffed over the holiday period. A study released this fall found that the kind of care patients received at hospitals was not to blame.

Early winter is the darkest period of the year, and another theory suggests that the lack of sunshine may increase the risk of a heart attack. Our bodies produce vitamin D when we are exposed to the sun's ultraviolet rays. Michael Holick, an endocrinologist at Boston University School of Medicine, says a vitamin D deficiency may be implicated in a number of diseases in adults, including heart disease and high blood pressure.

Then there is the football hypothesis. A study done in Britain found that the excitement of a close soccer match can literally be heart-stopping, and that the number of heart attacks jumped by 25 per cent when England lost to Argentina in the World Cup in 1998.

But if your team wins, it may actually be good for your heart. The death rate from heart attacks in France dipped when the French team beat Brazil that same year.

There is plenty of advice on how to avoid a winter heart attack that is easier to follow than only watching hockey games when you know your team will win. The experts say to eat right, dress properly when you go out in the cold, and get a flu shot. If you have heart disease or high blood pressure, check with your doctor before you shovel snow, and try to get regular exercise indoors.

If it's any consolation, Dr. Jong says, people in the tropics are at a greater risk of a heart attack when the weather gets hotter.