Clocks spring ahead and fall back when adjusting in and out of daylight saving time. A study published on Wednesday finds that heart attack rates do the same.

The research, based on heart attacks in Sweden, concluded that the chance of a heart attack goes up during the first three weekdays after the springtime shift to daylight saving time, possibly because of sleep deprivation.

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 Drs. Imre Janszky of the Karolinska Institute and Rickard Ljung of the National Board of Health and Welfare, both in Stockholm.

In a letter to the New England Journal of Medicine, they said the culprit may be the one hour of sleep lost in the spring, when the clocks are shifted ahead.

"The earlier wake-up times on the first workday of the week and the consequent minor sleep deprivation can be hypothesized to have an adverse cardiovascular effect on some people. This effect would be less pronounced with the transition out of daylight saving time, since it allows for additional sleep," they wrote.

The protective effect in the fall may last for just one day because, "Monday is the day when most of us will use this extra hour," Janszky said by e-mail.

During the shift to daylight saving time, women seemed more vulnerable to heart attacks than men. Men were more likely to be protected during the Monday in the autumn, the researchers said.

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