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Posted: November 11, 2008

Daylight Savings Time Shift Found to Affect Heart Attack Risk

We may have recently set-back our clocks to adjust away from daylight savings time, but Swedish researchers think we also may have set ourselves up to lower our risk of heart attack – until daylight savings time starts again.

Study results, published as a letter in the New England Journal of Medicine, found that, just as clocks spring ahead and fall back when adjusting in and out of daylight saving time, heart attack rates do the same.

Researchers at the Karolinska Institute in Stockholm examined data on heart attacks in Sweden around daylight savings time shift as far back at 1987 in concluding that the chance of a heart attack increases during the first three weekdays after the shift to daylight saving time in the spring, possibly because of sleep deprivation.

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However, they said heart attack risk declines on the first autumn Monday workday after clocks are set back one hour in the fall. On this point, researchers think the decline may be due to people taking advantage of the extra hour for sleeping.

Overall, heir results show that the number of heart attacks, on average, increases by about 5% during the first week of daylight savings time.

“There’s a small increase in risk for the individual, especially during the first three days of the new week,” says Dr. Imre Janszky, one of the Karolinska researchers. “The disruption in the chronobiological rhythms, the loss of one hour’s sleep and the resulting sleep disturbance are the probable causes.”

In the medical journal report, the research team wrote: "The earlier wake-up times on the first workday of the week and the consequent minor sleep deprivation can be hypothesized to have an adverse cardiovascular effect on some people. This effect would be less pronounced with the transition out of daylight saving time, since it allows for additional sleep."

Of note, the researchers said they found that, during the springtime shift to daylight saving time, women seemed more vulnerable to heart attacks than men. Men were more likely to be protected during the Monday in the autumn, they added.

Mondays have traditionally been cited statistically as the leading day of the week when heart attacks occur. Pulling from the daylight savings time findings, Janszky said: “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.”

Even though the increase and decrease in cardiac risk are relatively small for an individual, the Swedish team believes their study can improve the scientific understanding of how disruptions to diurnal rhythms impact on health.


“Roughly 1.5 billion people are subjected to these clock-shifts every year, but it’s hard to make any generalized statement about how many heart attacks they can cause,” added Dr. Rickard Ljung, another member of the research team and a member of the National Board of Health and Welfare in Stockholm.

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