Daylight Saving Time May Affect Heart

Study Shows Fewer Heart Attacks When Clocks Are Moved Back

Medically Reviewed by Elizabeth Klodas, MD, FACC on October 29, 2008

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.

The effect of the spring transition to daylight saving time on heart attack rates was slightly greater for women than men, and the fall effect was more pronounced in men than in women. And the effect was consistently more pronounced in people under age 65 than for those 65 and older.

Recent research links sleep deprivation to an increased risk for several heart attack risk factors including high blood pressure, inflammation, and obesity.

Northwestern University professor of preventive medicine Martha Daviglus, MD, says there is growing evidence that chronic sleep deprivation has a negative impact on the heart. But she is less convinced of the impact of isolated events like daylight saving time.

"I wouldn't want people to get the idea that losing one hour of sleep will cause them to have a heart attack," she says.

Sleep and the Heart

Daviglus, who is a spokeswoman for the American Heart Association, is conducting a study involving Hispanics with risk factors for heart disease that will include sleep analysis.

"You can't just focus on the amount of sleep people get," she says. "The quality of sleep is also very important."

People who take longer than 30 minutes to fall asleep at night or who are inexplicably tired throughout the day may have poor quality sleep.

The National Sleep Foundation estimates that 70 million Americans have sleep problems, with 40 million suffering from chronic sleep disorders.

According to the group's 2008 Sleep in America poll, the average American spends six hours and 55 minutes in bed each night, with six hours and 40 minutes actually sleeping. The National Sleep Foundation recommends getting seven to nine hours of sleep each night.

Show Sources


Janszky, I. The New England Journal of Medicine, Oct. 30, 2008; vol 359: pp 1966-1968.

Rickard Ljung, MD, PhD, scientist, National Board of Health and Welfare, Stockholm, Sweden.

Martha Daviglus, MD, professor, department of preventive medicine, Northwestern University Feinberg School of Medicine, Chicago; spokeswoman, American Heart Association.

National Sleep Foundation: "Sleep Facts and Stats" and "Sleep in America Poll."

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