The conventional view is that humans and other creatures around us live between periods of waking and sleeping. But it is not true. Many have mastered the art of hibernating, which allows them to spend quite a lot of their life in a mysterious state of suspended animation – sometimes more than half of it. What is hibernation, and is it something that humans might be capable of?

At the dawn of scientific enquiry into hibernation (from the Latin hibernus, pertaining to winter) in the mid-19th century, it was defined by Peter A Browne in an 1847 tract as ‘a natural, temporary, intermediate state, between life and death; into which some animals sink, owing to an excess of heat, or of cold, or of drought, or want of oxygen’. That’s a good first approximation. Now we know that – from dormice and bears, to hedgehogs, ground squirrels, bats and even tropical primates – hibernation is a very common phenomenon, found among representatives of at least seven different orders of mammals. It appears in many forms, which makes it difficult to define unequivocally, let alone imagine what it might look like in humans. As another early study points out: ‘we do not find that any two animals, however closely allied, hibernate in precisely the same manner, nor do individuals of the same species always hibernate alike’.

Nevertheless, there is a cluster of features typical of hibernation. The most parsimonious description would include reference to a controlled reduction in metabolism, reflected in a slowing down of many physiological and biochemical processes in the body. In sci-fi movies, hibernating humans are often depicted lying in pods, completely immobile and seemingly unconscious, and it is implied that their body temperature is very low – hence it is often called ‘cryosleep’ or something similar. Any mention of how exactly human hibernation is achieved in those conditions, or what triggers ‘awakenings’ from hibernation, is conveniently avoided, as if that were a trivial matter not deserving attention.

It could be forgivable to skip explaining how something as exotic as human hibernation happens. But it is sobering to think that sleep – a state so familiar to all of us, one we are perfectly capable of, on a daily basis – also remains a mystery. I am a sleep neuroscientist, and the focus of my laboratory is the origin and fundamental biology of sleep. I am convinced that sleep can be fully understood only when it is considered not in isolation but in juxtaposition with other states of being, such as hibernation. Yet research is still at a basic stage and there are so many things we don’t know about this aspect of life. How, for example, can we seriously talk about hibernation in humans, when it is a condition we also do not fully comprehend in other animals? And how can we understand sleep if we can’t clearly separate it from hibernation?

The revival of interest in hibernation in general, and human hibernation in particular, comes at the right time. The genre of science fiction is about imagining and predicting practical solutions for real-life problems when they cannot be solved with existing means. When the world is facing acute problems at a planetary scale, including climate change, technogenic disasters, wars, incurable disease, pandemics and mental health crises, and we are grappling with perennial questions, such as how to attain immortality (or at least extend high-quality life considerably), solve the mystery of consciousness or reach the far corners of the Universe, hibernation emerges as a potential opportunity, if not the only hope. From clinical applications to space travel, scientists, entrepreneurs, governmental agencies and even writers and artists turn to hibernation as a possible solution for our problems, those desires and anxieties we are unable to tackle with more down-to-earth approaches – or, at least, as a way to sleep through them and wake up when things are going better.

Scientists now agree that hibernation can be of two kinds – a seasonal, multiday profound suppression of metabolism, often lasting for months and occupying a good portion of an animal’s life, or else its shorter and milder form, a so-called daily torpor. The thirteen-lined ground squirrel (Ictidomys tridecemlineatus) is one typical example of a seasonal hibernator. When they perceive early signs of winter approaching, these squirrels gain weight and build long burrows in the ground where they courageously descend sometime in October, not to see light again until the following March, at least. Remarkably, the body temperature of these animals during hibernation can fall to sub-zero values, and heartbeats and respiration decrease to a tiny fraction of their normal rates. Not surprising, then, that hibernation has been described as a state in between life and death.

Hibernation epitomises what harmony with nature is about

One of the first lab studies of another ‘true’ hibernator, the Syrian hamster (Mesocricetus auratus), reports that, in preparation for hibernation, the animals built a nest that ‘was almost invariably a carefully made affair, so constructed that only the arched back of the animal appeared above the shavings’; then they ‘curled in a tight ball, with the nose tucked beneath the tail’. Their oxygen consumption and temperature would drop concurrently during the process of entering hibernation, reaching their minimal values 3-4 hours later. Djungarian hamsters (Phodopus sungorus), originating from southwest Siberia and northeast Kazakhstan, are instead an example of daily torpidators. When kept at about 6°C ambient temperature, the pelage, or fur, of these animals changes from grey-brown to white, and their body temperature falls to about 20°C; these bouts of so-called ‘daily torpor’ typically last for only a few hours.

Hibernation epitomises what harmony with nature is about – it is defined, by and large, by the amount and rate of exchange of matter, energy and information between inside and outside. Among the key triggers for preparing to enter and entering the state of hibernation are shortage, actual or perceived, of food and light. In response to a shortening day, some animals lose weight and others accumulate food stores, hiding from greedy competitors in secret places outside, or creating energy reserves within their own body – both to be consumed sparingly over the winter. They have to be very precise about this, as emerging too early from hibernation to seek more food means they may freeze or starve to death.

To test how animals deal with energy demands, scientists designed what they called a ‘work for food’ experimental protocol,........

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