A massive spike in heart attacks right around Christmas and New Year’s has confounded scientists, but a new study claims to have ruled out a cause. Many had theorized that these heart attacks happen due to the weather, but the study, published in the Journal of the American Heart Association, claims that it still happens at this time of year in New Zealand, despite the fact that it’s summer there when the holidays take place.

There is still an uptick of 4 percent in cardiac-related deaths in New Zealand during the holidays, according to a statement from the University of Melbourne. That forces scientists to examine other possibilities for the spike, and there are quite a few of them.

One of those possibilities is an increase in travel during the holidays. People end up in an unfamiliar area where they don’t know the local medical facilities, or they may choose to forego care altogether so that it doesn’t interfere with their time off. It may also have something to do with a drastic change in diet, or even stress levels. Hospitals may not be as well staffed during the holidays.

Scientists have seen similar spikes in cardiac-related deaths for religious groups around important holidays as well. Postponing care might play a role in these deaths.

“The Christmas holiday period is a common time for travel within New Zealand, with people frequently holidaying away from their main medical facilities,” lead author and researcher at the Centre for Health Policy at the University of Melbourne, Josh Knight, said in a statement.

“This could contribute to delays in both seeking treatment, due to a lack of familiarity with nearby medical facilities, and due to geographic isolation from appropriate medical care in emergency situations.”

Another possibility  is that terminally ill patients hang on for a few more days.

“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,” Mr Knight said.