Moving the clock ahead in the spring provides a pleasant extra hour of sunlight at the end of the day, but according to Swedish researchers it may have an unpleasant side effect: an increase in heart attacks.

Their study, published Oct. 30 in The New England Journal of Medicine, used 20 years of data from the Swedish registry of heart attacks to compare the rates in the days after the clock changes with those on corresponding days in the two weeks before and the two weeks after the change.

The lost hour in the spring was followed by a 7 percent increase in heart attacks in the first three days of the week, and a 5 percent increase for the entire seven days. There was no significant increase in the week after the time change in the fall.

Dr. Rickard Ljung, a co-author of the study and an assistant professor of epidemiology at Karolinska Institute, said it was well established that long-term sleep deprivation was a risk factor for heart attack, and added that more heart attacks occurred on Mondays than any other day of the week. The added effect of the clock change is small, he said, but it “could be harmful to people who have other risk factors.”

“Maybe a later start on Monday morning would be beneficial,” he continued. “Sleep is good for you.”