Turning your clock back may be good for your heart.
Swedish researchers looked at 20 years of records and discovered that the number of heart attacks dipped on the Monday after clocks were set back an hour from daylight saving time to standard time, possibly because people got an extra hour of sleep.
But moving clocks forward appeared to have the opposite effect. There were more heart attacks during the week after the start of daylight saving time, particularly on the first three days of the week.
"Sleep — through a variety of mechanisms — affects our cardiovascular health," said Dr. Lori Mosca, director of preventive cardiology at New York-Presbyterian Hospital, who was not involved in the research. The findings show that "sleep not only impacts how we feel, but it may also affect whether we develop heart disease or not."
The study was described in a letter published in Thursday's New England Journal of Medicine by Dr. Imre Janszky of the Karolinska Institute and Dr. Rickard Ljung of Sweden's National Board of Health and Welfare.
Janszky said he came up with the idea for the study after last spring's time change, when he was having problems adjusting.
"I was on the bus, quite sleepy, and I thought of this," said Janszky, who has done other research on sleep and health.
They took advantage of Sweden's comprehensive registry of heart attacks to see if the disruptions to sleep and the body's internal clock caused by a time change had any effect on heart attacks from 1987 to 2006. They compared the number of heart attacks on each of the seven days after the time shift with the corresponding day two weeks earlier and two weeks later.
Overall, in the week after "spring forward," there was a 5 percent increase in heart attacks, with a 6 percent bump on Monday and Wednesday and a 10 percent increase on Tuesday. In the week after "fall back," the number of heart attacks was about the same, except on Monday, which had a 5 percent decrease.
"The finding that the possibility of additional sleep seems to be protective on the first workday after the autumn shift is intriguing," the authors wrote.
Doctors have long known that Monday in general is the worst day for heart attacks, and they usually blame the stress of a new work week and increased activity. The Swedish researchers said their findings suggest that the minor loss of sleep that occurs at the end of ordinary weekends — with people going to bed later on Sunday and getting up early on Monday — might also be a contributing factor.
Last year, a study by American researchers found there were more pedestrian deaths during the evening rush hour in November than October as drivers and pedestrians adjust to the earlier darkness. They said the risk for pedestrians drops in the spring when clocks are set back and daylight comes earlier.
Daylight saving time in the United States ends this year at 2 a.m. Sunday. Sweden and the rest of Europe turned back their clocks last weekend. More than 1.5 billion people worldwide live in countries that use daylight saving time, the researchers said.
Sweden has a moderate rate of heart attacks and is at a high latitude, but Janszky said he would expect roughly the same results elsewhere.
Sleep can affect the heart through changes in blood pressure, inflammation, blood clotting, blood sugar, cholesterol and blood vessels, Mosca said. She suggested that anxiety from changes in routine may also be a factor, in addition to loss of sleep.
Dr. Ronald Chervin, director of the University of Michigan's Sleep Disorders Center, said this is a "sleep-deprived society," and he advises taking advantage of Sunday's time change and getting an extra hour of sleep.
In the spring, he suggests gradually adjusting to the one-hour loss by going to bed and getting up 15 minutes earlier for a few days before the time change.
"We spend a third of our lives sleeping and people forget how much effect it has on overall health," he said.