Washington, Oct 31 : Risk of heart attack increases with adjusting clocks in summers while turning the clock back during autumn reduces the danger slightly, shows a new Swedish study.
Scientists at Karolinska Institutet have examined how the incidence of myocardial infarction in Sweden has changed with the summer and winter clock-shifts since 1987.
The researchers found that adjusting the clocks to summertime on the last Sunday in March increases the risk of myocardial infarction in the following week, because of a lack of sleep during the time.
On the other hand, putting the clocks back in the autumn reduces the risk, but to a lesser extent.
The results indicate that the number of heart attacks, on average, increases by about five per cent during the first week of summer time.
"There's a small increase in risk for the individual, especially during the first three days of the new week. The disruption in the chronobiological rhythms, the loss of one hour's sleep and the resulting sleep disturbance are the probable causes," said Dr Imre Janszky, one of the researchers behind the study.
The researchers also observed that the readjustment back to winter time on the last Sunday in October, which provides an extra hour's sleep, results in a reduction in the risk of heart attack on the Monday.
However, the reduction for the whole week is comparatively less than the increase related to the summer adjustment.
The new study provides a conceivable explanation for why myocardial infarction is most common on Mondays-"It's always been thought that it's mainly due to an increase in stress ahead of the new working week. But perhaps it's also got something to do with the sleep disruption caused by the change in diurnal rhythm at the weekend," said Dr Janszky.
Despite the fact that the increase and decrease in risk are relatively small for the individual, researchers believe that the study can improve our understanding of how disruptions to diurnal rhythms impact on our health.
"Roughly 1.5 billion people are subjected to these clock-shifts every year, but it's hard to make any generalised statement about how many heart attacks they can cause," added Dr Rickard Ljung, another member of the research team.
The study was published in the prestigious New England Journal of Medicine.