Oct. 29 (Bloomberg) -- Turning back the clock and getting an extra hour sleep protects people against having heart attacks the next working day, according to a research letter in the New England Journal of Medicine.

The heart attack rate is generally lower on the Monday after the transition out of daylight saving time, while setting the clock ahead in spring appears to have the opposite effect, according to Imre Janszky from the Karolinska Institute and Rickard Ljung from the National Board of Health and Welfare in Stockholm.

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. These time shifts affect biologic rhythms, especially in the spring when more people were found to have heart attacks in the first three days after the change, researchers said.

``The most plausible explanation for our findings is the adverse effect of sleep deprivation on cardiovascular health,'' Janszky and Ljung said. ``Our data suggest that vulnerable people might benefit from avoiding sudden changes in their biologic rhythms.''

Daylight saving time benefits retailers and sporting events, and may cut traffic deaths and electricity use. It does, however, cause problems for dairy farmers because cows need to adapt to a new milking time, and for electronic instruments and medical devices, which must be programmed carefully to avoid causing harm, according to the study authors.

Sleep Deprived

The average sleep duration in Western societies decreased from 9 to 7.5 hours during the 20th century, which means people today are considered chronically sleep deprived, Janszky said. Monday is the day of the week associated with the highest risk of attacks because of the mental stress of starting a new work week and the increase in activity, the researchers said.

``Studies are warranted to examine the possibility that a more stable weekly pattern of waking up in the morning and going to sleep at night or a somewhat later wake-up time on Monday might prevent some acute myocardial infarctions,'' Janszky and Ljung said.

The researchers used data from the Swedish registry of acute myocardial infarction to calculate incidences of heart attacks during each of the first seven days after the transitions and the average in the two weeks before and after.

The Swedish Council of Working Life and Social Research, Ansgarius Foundation, King Gustaf V and Queen Victoria's Foundation and the Swedish Heart and Lung Foundation supported the research.

To contact the reporter on this story: Chantal Britt at cbritt@bloomberg.net

Last Updated: October 29, 2008 17:00 EDT