Many of us feel sleepy in the winter, but other species often take it a step further and hibernate. Dormice head underground to curl up tight, colonies of bats retreat to attics or caves, bears slumber in their dens, bumblebees burrow into soil and hedgehogs hunker down in their nests.
All kinds of animals hibernate, from insects and amphibians to birds and primates. It seems to do them good.
But while we may opt for the occasional duvet day as the days grow short and cold, humans cannot bed down for long periods. It all seems a bit unfair. Why, if it is so beneficial, do we not hibernate?
To work out why we do not, first we need to find out why animals do.
The most obvious reason for hibernating is to avoid the cold.
Hibernation is an extended period of energy-saving torpor. The body slows down and breathing, temperature and metabolic and heart rate all decrease.
It makes intuitive sense to do this in winter. Conditions are harsh and there is little food to be had, as fewer plants are growing. So many animals fatten themselves up during summer, then live off body-fat reserves until it is time to wake up.
In line with this, hibernation is more frequently found in the Northern Hemisphere. Most of the land in the Southern Hemisphere is near the equator, so winters are milder.
But this is not a hard-and-fast rule. Some species living in warm climes to hibernate, such as Sibree's dwarf lemur (Cheirogaleus sibreei) in Madagascar and the Southern African hedgehog (Atelerix frontalis) in Angola, Zimbabwe and other African countries.
What's more, hibernation is not confined to the cold months.
A 2015 study of edible dormice found that they continued hibernating even when cold conditions had ended. Some of the animals were hibernating underground for 11.4 months: nearly a whole year, and the longest hibernation period ever observed in the wild.
The major cause of death for a small mammal is being eaten
That is "a remarkably long absence," says co-author Claudia Bieber of the University of Veterinary Medicine in Vienna, Austria. In the study area it was only cold for 4-5 months. "So obviously there are other reasons."
The key issue seemed to be the behaviour of the local European beech trees. These sometimes have "mast years" in which they produce a bumper crop of seeds, which the dormice eat. Somehow the dormice could predict if this would happen, and if it did not, they stayed underground.
There are other sources of food, but the dormice need the extra beech seeds to reproduce. "They can eat fruit [like] apples, and it's enough for them to cope, but not to reproduce, or for the juveniles to gain enough body fat," says Bieber. "They skip reproduction these years and increase their survival by staying underground."
What's more, Bieber thinks the dormice had another reason to stay underground: predators.
By staying out of sight, the dormice avoided being hunted by birds of prey and owls. "The major cause of death for a small mammal is being eaten, and if you forage you are likely to be eaten by predators," says Bieber. A dormouse lurking underground will not give off any odour, so will be hard to locate.
We know that the survival rates in hibernation are close to 100%
Large white butterflies in southern Spain do something similar. Their pupae go into torpor for three months in summer, which is known as aestivation. A 2014 study found that this helps them avoid wasps that would otherwise parasitise their egg clutches.
"Until recently people thought hibernation was just about energy saving, a defence against cold weather and food shortage, and now we think it's more predator avoidance," says Thomas Ruf, also of the University of Veterinary Medicine in Vienna, Austria. "We know that the survival rates in hibernation are close to 100%."
Whichever reason is more important, hibernation seems to transform the way animals live.
Some of the dormice studied by Ruf and Bieber reached 12 years old, which is extraordinary for a small rodent. A wild mouse may live for just three months.
There is basically no fossil record and a lot of speculation
"Through hibernation they have managed to avoid predation," says Ruf. "When they do that, it starts to be important to invest in their own maintenance, in cell repair."
In a 2011 study, the team found that hibernators live slowly, invest more in survival and stay alive for a longer time. "All the hibernators seem to decrease their annual reproduction rate and increase their longevity," says Bieber. "Hibernation slows down the pace of life."
This means hibernation can affect how entire ecosystems work, by altering the rates at which animals reproduce and forcing predators to seek alternative prey.
But while we have some good ideas about how and why hibernation evolved, we have no idea when.
"There is basically no fossil record and a lot of speculation," says Ruf.
It has been suggested that dinosaurs living near the North Pole might have hibernated to survive the long winters. However, regular hibernation normally leaves traces in the microstructure of animals' bones, and a 2011 study of polar dinosaur fossils found no such evidence.
Humans evolved in equatorial Africa, deep in the tropics
Nevertheless, Bieber says hibernation and torpor are clearly millions of years old. For instance, all three major groups of mammals have evolved the techniques, and those groups diverged tens of millions of years ago. That suggests that at least some of the animals humans are descended from could hibernate.
However, we seem to have lost some of the key abilities. For instance, our hearts cannot work if they get too cold.
Our hearts contract in response to calcium, and if there is too much of it around the result is a cardiac arrest. Below a certain temperature, human hearts cannot remove excess calcium so they fail. "A human heart will stop if it cools down below 28 °C," says Ruf.
In contrast, the hearts of hibernating animals can still beat at a body temperature of 1 °C. Ruf says they have special pumps to remove calcium, which we lack.
But that simply raises a new question: why do humans not have these pumps? Our lifestyles may be a key factor.
Humans evolved in equatorial Africa, deep in the tropics where there is a fairly constant food supply. That means we would not have needed to hibernate to escape harsh conditions.
We are top predators that can tackle prey much bigger than ourselves
However, the more scientists look in the tropics, the more hibernating species they are finding, says Ruf. He has just discovered that pygmy slow lorises can hibernate for stretches of up to 63 hours between December and February.
That is particularly striking because lorises are primates, the same family that includes monkeys and humans. Until now only three primates were known to hibernate, all of them lemurs from Madagascar. The pygmy slow loris lives on mainland Asia, so the discovery indicates that primate hibernation is more widespread than anyone had expected.
All this means that our tropical origin probably reduced the odds that we would evolve the ability to hibernate, but by itself it would not rule it out.
Perhaps just as importantly, we are top predators that can tackle prey much bigger than ourselves. We probably never needed to hibernate to escape the threat of predators.
We might also be a bit big. The average hibernator is small and weighs on average 70g (2.5 oz). There are exceptions to this rule, bears being the most obvious, but they do not hibernate as "deeply" as other animals. In particular, they lower their temperatures less, because it would take so much energy to warm up.
Hibernators that lay down half a year do not suffer from any muscle waste or osteoporosis
Effectively, there is a checklist of factors that might prompt an animal to hibernate, and humans do not tick off a single item.
There may also be disadvantages to hibernation. For example, hibernating animals are basically without an immune system, so they are at risk of infection.
Hibernation also seems to affect memory. In 2001, Eva Millesi of the University of Vienna trained groups of ground squirrels (Spermophilus citellus) to walk through a maze. The groups that hibernated had lower retention and forgot what they had learned.
But that is not stopping scientists exploring the possibility of engineering human hibernation.
Anaesthesiologist Rob Henning of the University of Groningen in the Netherlands is working with NASA on exactly that.
There are reports that hibernators are protected from radiation damage
He says hibernation offers two key benefits to future astronauts. It would cut down on the volume of expensive supplies they need to take into space, and it would protect their health.
"One of the most annoying things for astronauts in space stations is they have to do physical exercise for 6 hours a day," says Henning. Otherwise, their muscles and bones atrophy.
But exercise in space is unpleasant, thanks to temperatures above 30 °C and air thick with carbon dioxide. If astronauts could hibernate, they would not have to do it. "Hibernators that lay down half a year do not suffer from any muscle waste or osteoporosis," says Henning.
Hibernation could also be a literal life-saver.
"There are reports that hibernators are protected from radiation damage, and this is really a big issue in space," says Henning. At the moment people can only stay in space for about a yea: beyond that, the radiation damage starts to significantly increase their risk of cancer. "If you were to hibernate people and they were to be protected, this would enable much longer missions," says Henning.
It might be that we can put astronauts into hibernation, but when they wake up they might not know who they are or what to do
It might also help with long-term colonisation. "In reality, it is not possible to have children in space, [because] eggs and sperm will be damaged so much that you will never get proper offspring."
This is literally the plot of a science fiction film, but Ruf and Henning are both fairly confident that it is possible. "If we understand the neuro-endocrine pathways, we may be able to put humans into hibernation one day," says Ruf. "We know that other primates, the Madagascan lemurs, are hibernators, so we may have the genetic makeup for the substances and pathways needed."
"I don't see why not," says Henning. "That's a little different than 'yes' but it's pretty close. Wherever you look animals hibernate… I don't see why we humans wouldn't fit in that picture somewhere."
Of course, if we did make it work we would have to face the same disadvantages animals do, such as memory loss. "It might be that we can put astronauts into hibernation, but when they wake up they might not know who they are or what to do," says Ruf.
Even if it is not possible to make a human hibernate, mimicking some of the processes involved could help treat human sickness.
When an animal hibernates, its organs appear diseased: lungs look asthmatic and brains exhibit signs of Alzheimer's. But when they wake up, the hibernators reverse these changes and everything returns to normal. Henning compares it to drinking 10 cans of an energy drink, so rapid is the return to good health.
It seems hibernation could be good for us
For example, a 2008 study revealed that polar bears do not lose bone mass during hibernation – the only mammal known not to do so. Wild pregnant polar bears were found to weather a six-month hibernation better than brown bears. We do not know how, but understanding the mechanism could help with the study and treatment of osteoporosis in humans.
The Dutch pharmaceutical company Sulfateq is investigating chemicals "based on the protective effects of nature’s hibernation mechanism". One such compound, SUL-121, has been tested as a possible treatment for a lung disorder called chronic obstructive pulmonary disease.
So far these treatments are either theoretical, or at a very early stage. But it seems hibernation could be good for us, even if we never manage to do it ourselves.