Many people dread the time changes that come with daylight savings time but a Swedish study, published in the New England Journal of Medicine, has found a healthy aspect that occurs when daylight savings time ends each year and reverts back to normal time – about 5% fewer heart attacks occur on the Monday after the end of daylight savings time. And the heart-attack rate for five of the six following days is reduced, too.

This heart-healthy bonus does not carry through to the springtime beginning of daylight savings time, though, according to Drs. Imre Janszky and Rickard Ljung. Their recent study revealed a spike of 6% to 10% in the number of heart attacks on the Monday, Tuesday, and Wednesday that follow the beginning of daylight savings time.

Jansky, of the Karolinska Institute, and Ljung, of the Swedish National Board of Health and Welfare, explored the medical records of Swedish people having heart attacks that resulted in hospitalization or death during the periods immediately surrounding the two annual daylight savings time transitions.

Using records dating from 1987 to 2006, the team determined the average number of people having heart attacks on a Monday in autumn was 2,140 except on the Monday after daylight savings time ended. On this particular Monday, the average number of heart attacks dropped by about 5%, to only 2,038. The rate of heart attacks occurring on five of the six days following the time change dropped enough to be apparent but not enough to be statistically significant.

No exact reason is known for the drop in heart attacks on this particular Monday but Dr. Ralph Brindis suggests sleep deprivation. Brindis, a vice president at the American College of Cardiology, says the Swedish study is “fascinating” and shows how significant losing just an hour of sleep can be.