Pure Pedantry

This Sunday you are slightly less likely to have a heart attack.

Swedish researchers, publishing in the NEJM, looked at a registry of heart attacks from 1987 to 2006. They found that the incidence of heart attacks slightly increases for the three days following the Spring daylight savings time where we lose an hour. The incidence of heart attacks slightly decreased on the day after Fall daylight savings time:

We used data from the Swedish registry of acute myocardial infarction, which provides high-quality information on all acute myocardial infarctions in the country since 1987. The incidence ratios, as measures of relative risk, and exact 95% confidence intervals were calculated.

The incidence of acute myocardial infarction was significantly increased for the first 3 weekdays after the transition to daylight saving time in the spring (Figure 1A). The incidence ratio for the first week after the spring shift, calculated as the incidence for all 7 days divided by the mean of the weekly incidences 2 weeks before and 2 weeks after, was 1.051 (95% confidence interval [CI], 1.032 to 1.071). In contrast, after the transition out of daylight saving time in the autumn, only the first weekday was affected significantly (Figure 1B); the incidence ratio for the whole week was 0.985 (95% CI, 0.969 to 1.002).

The authors attribute the change to getting one less hour of sleep during the Spring and one more hour of sleep during the Fall.

While the effect sizes are super-small and I am a bit skeptical about their ideas for mechanism (changes in pro-inflammatory cytokines due to changes in circadian rhythm), this is still a funny and cool story.

Hat-tip: NPR


  1. #1 penn
    November 2, 2008

    Besides the sleep issue, could it also be stress? I forgot about daylight savings time, and relieved and relaxed at having an extra hour. If I forgot in the Spring I would be anxious and upset about losing the hour. In the Spring it’s also likely to make you late for something which will increase your stress levels. Whereas, in the Fall you’ll have an extra hour to prepare.

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