Researchers at Good Samaritan Hospital in Los Angeles analyzed 2005-2008 death certificate data from seven U.S. regions with varying climates: Los Angeles County, Texas, Arizona, Georgia, western Washington state, Pennsylvania and Massachusetts.

In all areas, total deaths as well as deaths from heart attack, heart failure, cardiovascular disease and stroke rose by an average of 26 percent to 36 percent between the summer low and the winter peak.

"We found this to be surprising," said Dr. Bryan Schwartz, lead author of the study, presented at the annual scientific meeting of the American Heart Association. "We thought colder climates with a colder winter might have a higher increase in the wintertime or a prolonged increase in the wintertime, but that's not what we found."

The research was not designed to show why heart-related death rates rose across the board in the winter. But Schwartz theorized that people may "acclimate to their local climate," making the change between summer and winter temperatures more relevant than how low the temperature dips in winter.

He also noted that lifestyles tend to be less healthy in the wintertime. "Diet is not as good, people exercise less, they gain weight," he said, adding that less daylight is known to cause or worsen depression.

Higher rates of respiratory infection have also been shown to raise the risk of death from a heart-related issue, "which reinforces" guidelines for flu shots and pneumonia vaccines, Dr. Schwartz said.

(Reporting By Deena Beasley; editing by John Wallace)

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