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Heart Disease Health Center

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Daylight Saving Time May Affect Heart

Study Shows Fewer Heart Attacks When Clocks Are Moved Back
WebMD Health News

Oct. 29, 2008 -- This weekend brings an end to daylight saving time, and if you're lucky enough to get an extra hour of sleep when you turn your clock back Saturday night, a new study suggests that it might save your life.

When researchers in Sweden examined the impact of daylight saving time on heart attack rates in that country, they discovered that people had slightly fewer heart attacks on the Monday after they set their clocks back in the fall and slightly more heart attacks in the days after they set their clocks ahead in the spring.

They presented their findings in a letter published in the Oct. 30 issue of The New England Journal of Medicine.

Study co-author Rickard Ljung, MD, PhD, says the results suggest that even small disturbances in sleep patterns may affect the heart.

"We know that Monday is the most dangerous day for heart attacks," he tells WebMD. "It has been thought that this is due to the stress associated with returning to work after the weekend, but our study suggests that disturbed sleep rhythms may be involved, and that the extra hour of sleep we get in the fall [after daylight saving time ends] may be protective."

Spring Forward, Fall Back

Ljung and colleague Imre Janszky, MD, PhD, of Stockholm's Karolinska Institute compared heart attack rates in Sweden between 1987 and 2006 in the week following daylight saving time to heart attack rates two weeks before and two weeks after the spring and fall events using a comprehensive national health registry.

They discovered a 5% increase in heart attacks in the first three workdays after clocks were set ahead for the beginning of daylight saving time in the spring and a similar decrease on the Monday after clocks were set back for the end of daylight saving time in the fall.

"That is not a big difference, but it was significant," Ljung says. It also may translate into sizeable numbers of individuals in absolute terms, given that 1.5 billion people are affected by daylight saving time shifts across the globe.

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