It's called the Christmas effect. For years, doctors have noticed a spike in deaths in the U.S. right around the Christmas holiday in countries that celebrate it.
Now researchers in New Zealand say they can cross off one possible reason: the weather.
They've found the same deadly effect of Christmas down under, where it's the beginning of summer on Dec. 25.
Josh Knight of the University of Auckland and colleagues studied all the deaths in New Zealand from 1988 to 2013.
"We found evidence of a Christmas holiday effect in our of medical facility's cardiac deaths, with an excess event rate of 4.2 percent leading to about four additional deaths per annum," they wrote in their report in the Journal of the American Heart Association.
"A 'Christmas holiday effect' on mortality has been established in the United States, with spikes in deaths from natural causes at both Christmas and New Year's Day," they wrote. They are citing a 2004 study that found just under 5 percent more deaths over the end-of-year holiday period.
"In the United States, however, the Christmas holiday period coincides with the coldest period of the year, when mortality rates are already seasonally high because of low temperatures and influenza," they noted.
Not in New Zealand, where it's always mild and where Christmas and New Year coincide with the start of summer.
So what's going on?
"Various possible explanations exist for a mortality holiday effect, including the emotional stress associated with the holidays, changes in diet and alcohol consumption, less staff at medical facilities and changes in the physical environment (eg, visiting relatives)," they wrote.
They think the two most likely explanations are delayed medical care and "displacement of death." This means holding out until the holidays are over — something that studies suggest people who are otherwise dying may be able to do.
"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 and colleagues wrote.
It'll take more study to tell for sure.