NEW YORK (AP) -- Turning your clock back on Sunday may be good for your heart.

Researchers noticed a reduction in heart attacks in Sweden on the Monday after clocks were moved back.

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, possibly because people got an extra hour of sleep.

But moving clocks forward in the spring 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.