April may be the cruelest month, per Eliot, but it isn’t the deadliest. That honor goes to January.

The seasonality of death – more deaths in the winter, fewer in the summer – is a well-established and long-running public health mystery.

A few years ago, a sociology professor named David Phillips examined 57 million death certificates issued between 1979 and 2004 and made an interesting finding: Not only do more people die in the winter months, but New Year’s Day is actually the deadliest day of all. Phillips plotted the millions of deaths from natural causes according to the day of year on which they occurred.

You get a sort of wave of death that ebbs in the late summer months, rises sharply through December, and hits a peak exactly on Jan. 1 before subsiding more gradually through the beginning of the year.

The general contours of this wave are well known. Lately, a typical January has seen 40,000 to 60,000 more fatalities than the typical August or September.

The burning question: Why? Does it have something to do with suicides or car crashes or drinking around the holidays? Nope: “This pattern turns up in every natural cause of death, but not for external causes like auto accidents,” Phillips said in an interview. “It’s hard to understand why that would be.”

In other words, the deaths are being driven generally by illness, disease and old age, rather than by accidents or homicides or any other non-health-related cause.

Not only do we not know what’s driving the seasonality, we’re not even trying to figure it out as far as Phillips is concerned. Nobody he’s aware of is doing any research into it. “It’s not only a mystery, but a mystery that people haven’t even tried to engage with,” he said.

Phillips’ research has turned up some limited but tantalizing clues. When he restricted his 57 million death certificates to only people who had died in an emergency room, the spikes on Christmas and New Year’s Day became hugely exaggerated. “There are more emergency department deaths on Christmas, the day after Christmas, and New Year’s Day than on any other day of the year,” he wrote. And those jumps in fatality were higher than what you’d expect simply because of the general seasonality of death.

So he started looking for causes. It doesn’t seem to have anything to do with the weather, or with people being indoors more, because the holiday spikes are even bigger in Southern states, where it’s warmer. The spikes are just as big if you exclude people who died from alcohol or drugs, or people who died from pneumonia or the flu.

He did come up with a couple of hypotheses that he wasn’t able to eliminate: There has been some research showing that “people postpone going into the ER around the holidays because they want to be with their family,” he said. In situations where minutes or seconds can mean the difference between life and death, some folks may be, say, feeling a twinge of chest pain at the start of dinner but not telling anybody about it until afterward.

Another potential reason is that hospitals may be understaffed around the holidays, and their most experienced workers may be taking time off. That could lead to reduced performance and reduced ability to save lives.