This article first appeared in Issue 5 of our free digital magazine CURIOUS.
When there’s a nip in the air and the long, cold nights are drawing in, animals sensibly get under the metaphorical covers and don’t surface until spring. So why, exactly, do humans stay awake? And – more importantly – do we really have to?
What is hibernation?
We all know what hibernation is, don’t we? It’s probably one of the earliest “science” facts we learn in life: every winter, instead of facing the cold and dreary weather ahead of them, certain animals opt to bulk up, hunker down, and nestle in for a nice, deep sleep until the sun comes out again.
But like seemingly every piece of science we learned in elementary school, “hibernation” is more complicated than that. For one thing, it’s not one thing at all: there are varying levels of dormancy in the animal kingdom, ranging from the one-day bouts of torpor experienced by animals like mice or bats to transformations so extreme as to be almost unbelievable.
“For instance, when arctic ground squirrels hibernate, they get too cold to perform an EEG [electroencephalogram],” explains Kelly Drew, a researcher in hibernation biology at the University of Alaska, Fairbanks, and director of the NIH Center for Transformative Research in Metabolism. With a core body temperature lower than the freezing point of water, “it looks like they’re in a coma,” she told IFLScience.
Not every winter napper is so dramatic about it. Brown bears, one of the most famous hibernators thanks to the annual pageantry of Fat Bear Week, experience a drop in body temperature of less than about 5°C (9°F) – and contrary to how we imagine hibernation, they wake up quite often during those months inside.
This might leave you wondering: if both these extremes count as hibernation, then what exactly is the unifying feature here?
“Technically, [torpor] refers to a regulated state of reduced metabolism, meaning the chemical reactions in an organism’s body that keep it alive slow down,” explained Vladyslav Vyazovskiy, an associate professor of neuroscience at the University of Oxford, in an article for The Conversation.
“Heart rate, breathing and energy consumption all dramatically decrease, body temperature can also fall.”
It’s these technicalities that separate hibernation, which is the extended period of torpor found in some warm-blooded animals, from brumation, the dormant state experienced by certain reptiles like turtles or leopard geckos during colder weather. It is, on its surface, virtually indistinguishable from hibernation – but metabolically it’s very different, being induced by external rather than internal processes.
“Obligate hibernators, like ground squirrels, are driven to hibernate by an endogenous circannual rhythm,” Drew told IFLScience. “Facultative hibernators, like hamsters, respond to shortening day length. In both cases, hibernation is linked to reproduction and resource availability.”
Tucking in for the night
If we want to extend hibernation to humanity, we should probably know what we’re signing up for. What, precisely, is going on as an animal settles in for the winter – and could a human body really cope with such a long period of, to put it bluntly, nothing at all?
“Controlled hypothermia and metabolism are already widely used in clinical practice,” explained Vyazovskiy. “[It’s used] during cardiac surgery and to protect tissues from damage when blood flow is reduced, such as after a stroke.”
There are clear benefits to the technique: a reduced metabolism and body temperature means the cells in the body require less oxygen to survive, for example. “But the key difference is that animals [seem] to ‘know’ the way to safely and spontaneously enter torpor,” he continued. “Lowering a human’s body temperature by blocking their natural thermoregulation requires the aggressive use of drugs.”
As it stands, replicating true hibernation or torpor in humans is a little out of reach – and that’s for quite a few reasons.
We know what happens to an animal’s body as it retreats for the winter: it’s essentially a two-step process, Drew explained, with defined periods of pre-hibernation and torpor.
“[In] the pre-hibernation season, gonads regress, the drive to sleep increases, the thermostat is turned down, metabolic rate decreases,” she told IFLScience. “The second switch drives the animal into prolonged torpor, [in which] thermogenesis (heat production) is turned down significantly… [which] then contributes to further energy savings.”
But what we don’t know exactly is how this all occurs. Is it a “bottom up” process, where the main biological changes begin at a cellular level? Or is it more “top down” – starting with a hormonal response or a signal from the nervous system, and rippling down to the smaller metabolic processes?
Evidence has been found for both these possibilities, with little in the way of any conclusive answers. And the mysteries don’t stop there – there are some things about hibernation that, frankly, don’t make sense at all.
Hibernation on Mars?
By the time a bear emerges from hibernation, it will be significantly smaller and lighter than when it started. That’s not surprising, it has spent up to seven months near immobilized, without eating, drinking, peeing, or pooping. But the loss isn’t quite as catastrophic as you might think – and that offers some intriguing possibilities for the future.
“Bears exit their den healthily in spring with only marginal loss of muscle mass,” said Professor Alexander Choukér, co-author of a paper on the European Space Agency's (ESA) hibernation strategy for deep space missions. “It only takes them about 20 days to be back to normal. This teaches us that hibernation prevents disuse atrophy of muscle and bone, and protects against tissue damage.”
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Hibernation is quite the hot topic in the astronautics industry right now. As humanity looks hungrily towards our red neighbor, many experts have hit upon induced hibernation – or, more likely, torpor – as the solution for myriad problems that would beset any future missions to Mars.
A round trip to the Red Planet would likely take around 18 months, during which time any human passengers will probably want to eat, or move around. Spending so long in space could also have serious implications for any astronaut's physical health. Even outside of practical concerns, human hibernation might be worth it for the psychological protection it could provide.
“If humans could [hibernate]… it could increase our chances of making humans a spacefaring species,” Gary Hardiman, senior author of a study inducing torpor in zebrafish, said in a statement. “It would… cut down on psychological stress. The change to [astronaut] metabolism would stop them requiring food, oxygen or water, and there is a possibility it would protect their muscles from wasting due to the effects caused by radiation and microgravity.”
The hazards of hibernation
By now, we’re sure there are some of you out there wondering where you can sign up for a human hibernation study. But don’t be too hasty.
Like ESA, NASA has dedicated research regarding human hibernation protocols – but even it warns that hibernation would have drawbacks: it would require our bodies to be fed via a tube inserted through the abdomen, and, as it stands, would likely require heavy sedatives to squash our natural shivering reaction.
And you can forget about feeling well-rested afterwards. Hibernation is not the same thing as sleep – it “can be thought of as an extension of sleep,” Drew told IFLScience; “in fact, animals have to be asleep to enter hibernation – they go from slow wave sleep, into hibernation, and then back through slow wave sleep again to wake up.”
Perhaps most concerning – especially if you happen to be 94 million kilometers (58 million miles) away from home at the time – is the effect such an extended period of unconsciousness would have on your brain. That’s particularly difficult to quantify because we don’t actually know what it would be.
“Torpor affects the brain… by reducing and reorganizing the synaptic connections that are the basis of our memories,” explained Vyazovskiy. “If we wanted to induce human hibernation it would be vitally important to investigate further how memories are retained over a long period of torpor.”
Human hibernation: could it be possible?
Regardless of any benefits or drawbacks, there’s one question at the heart of this we have yet to answer: could humans actually learn to hibernate?
“We think humans can hibernate,” Drew told IFLScience. “Experienced yogis can suppress [their] metabolism to induce hibernation-like states.”
If that surprises you, consider this: According to Dr Sandy Martin, author of a study last year investigating the particular gene expressions involved in mammal hibernation, it is simply so ubiquitous across the evolutionary tree of mammals that it's probably just hard-wired into our genetic code. “The common ancestor of all mammals was [probably] a hibernator,” she recently told New Scientist. “It’s possible we all have the genetic hardware.”
In other words, hibernation in humans may not be a case of “learning”, but “remembering” how to do it. Indeed, we may not even need to stretch that far back, evolutionarily speaking. There’s “even some evidence,” albeit divisive, Drew told IFLScience, “suggest[ing] that prehistoric humans may have hibernated.”
To infinity, and zzzzzzzzzz
So, could humanity’s future among the stars rest in a biological process we left behind in fossils? Perhaps. The truth is, Drew told IFLScience, that we’re “still far from understanding” the nuances of hibernation, both past and future.
“Hibernation occurs across a wide and diverse spectrum of animals with no clear evolutionary lineage,” she pointed out. As to exactly why some animals go in for a winter-long naptime while others don’t, “we do not know,” she admitted.
Which sort of brings us to the crux of the matter: even if humans could learn to hibernate – would we want to? After all, it developed as a way for animals to survive the cruel and unforgiving winter; humans, with our unprecedentedly cushy lives, and our year-round access to things like hot food and central heating, may find ourselves having to give up our creature comforts if we really want to justify hibernation to our genome. Would that really be worth it?
Perhaps, if the hopes of NASA and ESA are realized, hibernation really does have a place in the future of humanity. But it will take a lot of hard work and research to get there.
As for the snoring astronauts of years to come, we can only imagine how different their lives will look from our own – and how similar they may look to those of certain other, more gloriously rotund members of the animal kingdom.
“Oh, bears, definitely,” Drew tells IFLScience. “Bears are the perfect model for human hibernation – they’re a similar size to us, they don’t get too cold like ground squirrels… bears are absolutely what we’d want to mimic.”