New research suggests that certain times of year or days of the week could affect the incidence of myocardial infarction (MI).

The observational study, published in the American Heart Journal, included more than 156,000 people from Sweden. Initial results found that the incidence of MI was higher during the winter holidays and on Mondays, compared to weekends and the summer.

The researchers, who are from Uppsala and Umeå Universities, both in Sweden, think the high incidences MI is linked to stress since Mondays and holidays are associated with feelings of being overwhelmed and busy.

When the researchers controlled for temperature, air pollution and air travel, the findings were consistent. Previous research has shown that stress-triggering events, like earthquakes and World Cup games, can cause MI.

"This is the first study that investigates these culturally defined time-periods in the Swedish population with unselected, high-quality data,” said John Wallert, a PhD student and lead author on the study, in a statement.

Wallert and his team used symptom start and hospital admission dates with predefined hypotheses to come to their conclusions.

“Previous studies have often lacked symptom start and discussed their results in terms of a delay in seeking appropriate care or delay of registration as explanatory for the MI rate variation over time,” Wallert said. “We found that such factors seem to explain only a part of the variation. Our study seems to suggest that psychosocial demands on behavior influences basal biological systems, even to such an extent that they may be potential triggers for MI. The associations of calendar periods with MI rates are surprisingly robust.”