Here's a new reason to look forward to the switch back to standard time Sunday morning: it may lower your chance of suffering a heart attack.

Heart attacks decrease by 5 percent the first Monday after the time change, and by 1.5 percent over that week, according to an analysis in this week's New England Journal of Medicine. The findings are based on 20 years of data from a Swedish registry of nine million residents.

The springtime transition to daylight saving time poses more of a health hazard: Heart attacks increase by 5 percent over the first week after clocks are pushed back an hour, spiking by 10 percent on that Tuesday, epidemiologists at the Karolinska Institute in Stockholm found.

The reasons aren't clear, but study co-author Imre Janszky suspects that changes in sleep may play a role. Although there is little data on the cardiovascular effects of the kind of short-term sleep deprivation that occurs over the average five days it takes to adjust to moving clocks ahead in spring, chronic sleep deprivation is associated with increases in blood pressure, heart rate, blood clotting, and C-reactive protein, a marker of inflammation associated with deadly heart attacks, Janszky says.

The time clock transition causes "a complex disruption of our body rhythms," Janszky says, "so there are plausible reasons for sleep deprivation as a trigger for heart attack."

One more reason to suspect sleep deprivation as a culprit: the incidence of heart attacks shifted around the time change only among Swedes under 65, the official retirement age there. Research published in 2005 by Dutch scientists showed that heart attacks are more common on Mondays, possibly because of the stress of beginning the work week—and the readjustment to getting up early.

"Our results suggest that this shortness of sleep contributes to the increased risk on Monday—except for that Monday [after the switch to standard time] when we have that extra hour we can use for sleep," Janszky says.

(Image by iStockphoto/Lisa Gagne)