This spike, identified by sociologist David P. Phillips while combing through death certificates in the United States, has been confirmed in subsequent studies using Centers for Disease Control and Prevention and other large data sets. It holds true for all ages except for children and for numerous conditions including heart disease, respiratory problems and cancer.

Speculation about why this happens has ranged from in-law stress to the excesses of eating all that fruitcake and drinking all that eggnog, but the prevailing theory accepted by many doctors and researchers has always been about the weather. The colder it is, the more vulnerable people's bodies might be to having a complication from a heart attack, flu or other ailment that they can't recover from.

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The logic may be perfectly reasonable, but it's likely wrong. According to a new study published in the Journal of the American Heart Association on Thursday, the rise in deaths during the holidays doesn't appear to have anything to do with the cold winter months.

To try to separate the season from the weather, researchers looked at death data from the opposite side of the world — in New Zealand, which is in the Southern Hemisphere, where Christmas falls in the summer.

The study focused on the 197,109 deaths due to cardiac issues in a 25-year period from 1988-2013 and found a jump in heart-related deaths during the holidays, a pattern similar to that seen in the United States. The methodology they used for the analysis involved calculating an expected number of deaths based on month and season and found a 4.2 percent increase from Dec. 25 to Jan. 7. Unlike in the United States, the holiday season doesn't represent the absolute peak of cardiac deaths, which happens in June, July and August during the New Zealand winter.

The researchers said the findings clearly suggest that the “holiday effect” in deaths is separate from any “winter effects.” In the graphic below look at the jumps in green (“Pre-Christmas”), pink (“Christmas”), and blue (“Post-Christmas)” as compared to the purple immediately before and after the holidays.

Josh Knight, a research fellow at the University of Melbourne in Australia, and his co-authors wrote that the paper “helps reaffirm that there is no apparent correlation between any observed Christmas holiday effect and the impact of temperature or seasonality.”

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The researchers also noted that, “Persons who die from a cardiac event, in or out of hospital, during the Christmas period are younger by almost a year than those who die from the same or similar condition during the remainder of the year.”

Knight said in a statement that this still leaves many other theories unaddressed, including the possibility of reduced staffing at medical facilities or even that patients hold back on seeking medical care during the season as being factors causing the jump in deaths.

Another idea Knight described is more philosophical.

He explained it could be that terminally ill patients may be hanging on to spend the holiday with their loved ones and then feel ready to let go and say goodbye.

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“The ability of individuals to modify their date of death based on dates of significance has been both confirmed and refuted in other studies, however it remains a possible explanation for this holiday effect,” Knight said.