A Swedish study has revealed that adjusting the clock forward in summertime increases the risk of a heart attack, while turning the clock back during autumn reduces the risk slightly.
The study, conducted by researchers at the Karolinska Institute in Stockholm and published in the New England Journal of Medicine, showed that in the week after clocks are reset for summertime there is a five-percent increase in heart attacks, which the scientists attribute to sleep deprivation.
"In the spring, there is a clear statistically significant increase in the risk of suffering a heart attack" in the week after clocks are set forward, which in Europe happens on the last Sunday of March, Karolinska researcher Imre Janszky told AFP.
In the autumn however, when clocks in Europe are set back an hour on the last Sunday in October, "the picture is not so clear," he said, pointing out that the effect then is only clearly seen on the following Monday when there are five percent fewer heart attacks on average.
According to the study, which is based on data from a comprehensive register of all heart attacks in Sweden from 1987 through 2006, the reason for the increases and decreases in heart attack rates is closely linked to the effect the sudden time change has on sleeping patterns and biological rhythms.
"Some people find it quite difficult to adjust to it, and the sleep quality and sleep duration is affected and it takes a certain toll," Janszky said.
"There is a growing body of evidence that problems of disruptions of the biological rhythms and sleep problems are connected to cardiovascular health."
While the shift in the spring is clear - cut, since it both disrupts the biological rhythm and discards an hour of potential sleep, the autumn shift features "two different mechanisms working against each other," Janszky said. "We still have a disruption of the chronobiological rhythm, but there is a possibility for some extra sleep," which appears to outweigh the negative, he added.
Monday has long been considered "the most dangerous day of the week" when it comes to heart attacks, something previous research has attributed to a sudden increase in activity and stress related to the week ahead.
Janszky however said the Karolinska study hinted at another explanation.
"During the weekend you often get up later and you go later to bed. If you go to bed later on Sunday but still have to get up early on Monday you will end up with a sleep deprivation," he said.
Some 1.5 billion people around the world are subjected to the seasonal clock changes each year, and while Janszky cautioned against drawing a general conclusion from the Swedish study he said he "would guess that roughly the same (heart attack patterns) would be seen in the whole world".
Asked whether jet lag while traveling could have the same effect as the seasonal clock adjustment, Janszky said it was unlikely.
"Studies on sleep find that the effect of daylight savings time is more than jet lag (since) when you travel you will change your social clock ... but the environmental signals will be in harmony," he said.
When we simply set the clock forward or back an hour however, the sun still rises at about the same time, but you are suddenly expected to shift your social habits and go to work earlier or later.
"This makes this shift more dangerous to some people," Janszky said.