Heart attack rates appear to be increasing during the daylight saving time according to a study by Swedish researchers. How is that possible? Well, according to the study, sleeping an hour less might affect cardiovascular rhythm in persons having heart problems.

On the other hand, 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, Dr. Imre Janszky of the Karolinska Institute and Dr. Rickard Ljung of Sweden’s National Board of Health and Welfare said. Their findings were presented in a letter published in the Oct. 30 issue of New England Journal of Medicine.

For the study, Dr. Janszky and Dr. Ljung 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.

Sleep deprivation has long been shown to be bad for the human body, especially for the heart, as it leads to high blood pressure and high rate which can cause dangerous clots. About 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.

According to the National Sleep Foundation, seven to nine hours of sleep each night should be enough to feel rested the second day and avoid health problems.