There is clearly a link between sleep and cardiovascular problems, Swedish researchers found. According to their study, the more you sleep the smaller your chance of having a heart attack is. And when do people sleep more? Usually when turning clocks back on every autumn.

The researchers found that the number of heart attacks dipped on the Monday after clocks were set back an hour, possibly because people got an extra hour of sleep. On the other hand, moving clocks forward in the spring appeared to have the opposite effect. There were more heart attacks during the week after the start of daylight saving time, particularly on the first three days of the week.

The study was the work of Dr. Imre Janszky of the Karolinska Institute and Dr. Rickard Ljung of Sweden’s National Board of Health and Welfare. They presented their findings in a letter published in the Oct. 30 issue of New England Journal of Medicine.

The researchers analyzed data on heart attacks for a period of 20 years between 1987 and 2006 and found that the rate of heart attacks increased 5 percent in the first week with 6 percent increase on Monday and Wednesday and 10 percent increase on Tuesday after clocks were set forward one hour in the spring. Oppositely, when clocks were set back one hour, the rate of heart attacks dipped 5 percent on Monday although the rate for the first week remained pretty much the same.

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

Five percent is not a big difference, but it is still significant, given that 1.5 billion people are affected by daylight saving time shifts across the globe. Daylight saving time is commonly used in the northern hemisphere to add an hour of daylight to the afternoons. The phenomenon appeared to have worse consequences on women, as they were more likely to have heart attacks compared to men. And the effect was consistently more pronounced in people under age 65 than for those 65 and older.

Recent research associates lack of sleep with an increased risk for several heart attack risk factors including high blood pressure, inflammation and obesity. Of course, not only the quantity of sleep matters, but also the quality as well. According to the National Sleep Foundation, 70 million Americans have sleep problems with 40 million suffering from chronic sleep disorders.

A poll of the organization this year found that the average American spends six hours and 55 minutes in bed each night, with six hours and 40 minutes actually sleeping. How much sleep is recommended? Well, the organization says seven to nine hours of sleep each night should be enough to feel rested the second day.