The risk of heart attacks changes when clocks spring ahead or fall back at the start or end of daylight saving time, according to a study published Thursday in the New England Journal of Medicine.

Setting the clock ahead in spring for the start of daylight saving time appears to increase the heart attack rates, possibly because of sleep deprivation, said the study based on heart attacks in Sweden.

But on the autumn Monday after clocks go back and people can get an extra hour of shuteye, the heart attack risk declines.

"Our data suggest that vulnerable people might benefit from avoiding sudden changes in their biologic rhythms," said Imre Janszky of the Karolinska Institute and Rickard Ljung of the National Board of Health and Welfare, both in Stockholm.

The culprit is probably sleep. Scientists have known that sleep deprivation is bad for the heart -- the body responds by boosting blood pressure, heart rate and the tendency to form dangerous clots.

Daylight saving time is commonly used in the northern hemisphere to add an hour of daylight to the afternoons. More than 1.5 billion people turn their clocks forward in the spring and backward in the fall.

The study shows that people under age 65 seemed more vulnerable to heart attacks than youngmen during the shift to daylight saving time.

Janszky said younger people may be affected more because they tend to be working and their schedules are not as flexible.

(Agencies via Xinhua October 31, 2008)