The days are getting shorter, the nights are drawing in, and the heating bills are beginning to bite. Winter is on the way.

For many mammals, that means only one thing: it’s time to find somewhere cosy and settle down for winter. Humans, it turns out, aren’t that different.

Whilst modern humans don’t hibernate, it’s possible some of our ancestors did, and we certainly slow down in the winter. Much like other mammals, we tend to give birth in the spring. The exact month changes depending on where we are in the world, but temperature plays a role in almost every geography.

It’s no wonder, then, that late autumn and early winter are so often referred to as “cuffing season.” And as the tailwinds of hot girl summer dissipate, more people than ever are looking for a match. 

But what drives this strange seasonality? Is there any science behind the meme?

To begin with, it’s no coincidence that cold and ‘flu rates skyrocket in winter. Part of this effect is due to the fact that we’re all more likely to be cloistered indoors, where viruses can circulate freely, but part of it is to do with the cold itself.

Spending less time outside means our levels of vitamin D start to plummet. For those of us in cooler climes, they may not have been high enough to begin with – about 40% of Europeans are deficient – but winter brings even less time spent in the sun, which compromises our immune systems.

There’s also the fact that cold air is drier than hot air, which means viral particles hang about for longer. Finally, the lipid (fat) coating that surrounds many viruses is stronger in colder climates, letting the viral particles enter our body more easily.

As we all know by now, infants and the elderly are most at risk from respiratory viruses (and most other bugs). Babies are protected by their mothers’ immune systems for a few weeks after birth thanks to a phenomenon called passive immunity. But this doesn’t last, and babies’ own immune systems are much weaker than most adults’. They’re uniquely at risk of infection, and what might be a cold or a passing bug in a healthy adult can be deadly for an infant (particularly one born pre-term, as about 10% of infants are). 

In the 21st century, medical science means that a huge number of illnesses are survivable. But this wasn’t true during the majority of human evolution. It makes sense, then, that as a species we’d evolve to give birth in warmer months, when viruses have less power.

But like all mammals, we can only afford to give birth in the spring or summer if we conceive in the autumn and winter – and that means finding a mate. For women in particular, whose biological burden dramatically increases with pregnancy, finding a partner to provide additional resources is a logical step. Some remaining hunter-gatherer groups do not preserve food itself, but store surplus in each other’s stomachs. When one person has an abundance of food, she shares it with the group, so that when another member is in a similar position, they will do the same. Human couples have worked like this since the beginning of our evolution, and the same effect is visible in many other animals. By pairing up, they can divide the labour required to bear and raise young. Perhaps most famously, penguins take it in turn to spend months away from their young, fishing until they have enough food to regurgitate for a family.

This makes all the more sense in winter: the time of year when, historically, food was at its most scarce. Just as with animals, the metabolisms of plants slow down in the cold and dark, and seeds become dormant. A few berries thrive – but the reason they do so is that they lack most of the nutrients peculiar to summer fruits, making them unpleasant and sometimes dangerous to eat.

Humans didn’t develop agriculture until about 12,000 years ago; a mere snip in the grand evolutionary timeline of our species. Even after farming became widespread, we still depended upon remnants of the summer’s bounty for colder months, and a bad harvest invariably spelled a hard winter. 

Other animals cope with winter’s hardship by gorging themselves when food is plentiful, and slowing their metabolisms down dramatically when food is scarce. This typically results in hibernation, or at least dormancy, during the winter.

Humans also exhibit slowed metabolism, reduced social interaction, and diminished ability to carry out daily tasks when deprived of food. Indeed several writers in the 19th and 20th centuries testified to Russian populations in famine districts like Pskov, east of Latvia, who spent winter in "almost uninterrupted sleep," referred to variously as loijka and lotska, in order to survive the cold.

Herodotus, writing in the 5th century BC, heard rumours of a population who “slept for half the year,” locating them roughly in what is now western Russia. It is possible, although by no means well-evidenced, that the practice of sleeping heavily (if not wholly) through winter months to slow one's metabolism has survived millennia of icy weather in some Europe-facing parts of Russia.

Archaeological evidence of real human hibernation, however, suggests we tolerated it badly. 430,000 years ago, there existed a Spanish species of hominid that archeologists suggest slept during the winters of the Elster glaciation, historically referred to as one of the Ice Ages. The species was plagued by rickets, bone lesions, tumours, and thyroid problems caused by lack of sunlight. Their puberty stopped and started each year with the seasons, and kidney problems were near-universal. Many other mammals, including bears and rodents, face similar problems if they don’t convert enough food to fat before winter then many of them die mid-hibernation. These are problems biologists may need to overcome if humans are to spend more than a few years in space.

Whilst we don’t hibernate like some of our ancestors, humans do become more insulin-resistant in winter, and our average blood pressure increases. Our waist circumferences also increase as we store more calories as fat. For some of us, the symptoms spread to our mood: Seasonal Affective Disorder, or SAD, is associated with craving carbohydrates, and hypothesised by some researchers to be a precaution our bodies take to make sure we save up enough energy for the cold winter months. 

It’s no wonder, then, that as summer warmth trails away we begin to crave company. Facebook’s data team laid the pattern bare: in winter, we pair up; in summer, we break up. Today, most of us are lucky enough to have houses, but an instinct for company when the cold begins to bite is conceivably a natural outgrowth of history. Sharing warmth, like sharing food, was once essential to our survival.

With all that in mind, it’s no wonder many of us feel drawn to settling down as the afternoons darken. Hot girl summer – the time for meeting new people, trying out potential partners, and enjoying the season of plenty – yields to the pumpkin-spiced hues of autumn and, eventually, the chill of midwinter.

If you’re looking for someone to help you avoid the judgement of elderly relatives this Christmas, we’ll be happy to help.