As winter approaches, bears and other hibernators — from butterflies to box turtles — begin preparing to clock out until spring.
Us humans, on the other hand, find ourselves stuck all-too-consciously contemplating the frozen months ahead. In winter, everything feels twice as hard: getting dressed, shoveling snow, averting existential meltdown. Wouldn’t it be nice to ring in the new year, promptly tunnel down some dark burrow, and curl up to wait for better days?
There’s no clear path to human hibernation, or any guarantee that it’s possible. But researchers have begun to tease apart the science of the long nap, inspired by its potential to revolutionize space travel and medicine — and save us from the icy hell that is January.
What Is Hibernation?
Hibernation is a survival strategy used by certain animals during harsh winter conditions. Animals significantly reduce their metabolic activity and body temperature to conserve energy.
Which Animals Hibernate?
While the term "hibernation" is often used for various winter dormancy states in vertebrates, true hibernators include animals like some fishes, amphibians and reptiles that have near-freezing body temperatures during winter, along with a few mammals like bears. However, these mammals don't experience the same degree of temperature drop and can be easily awakened, so they are not considered true hibernators.
Why Do Animals Hibernate?
The main advantage of hibernation is that you need less food and water, which explains why so many animals do it during the portion of the year when resources are scarce.
Brian Barnes, former director of the Institute of Arctic Biology at the University of Alaska Fairbanks, explains that hibernation “is best viewed as an adaptation to anticipated famine and not to winter or cold, per se.”
In other words, hibernation allows animals to stretch their fat reserves farther by reducing their body’s temperature and life-sustaining processes to a minimum. Their heart rate and breathing drop by as much as 98 percent.
Why Don't Humans Hibernate?
Humans don't hibernate because our ancestors evolved in a tropical environment where food was plentiful year round, they had no use for this survival strategy, and thus never acquired the biological mechanisms for it. Plus, when they eventually migrated to cooler climates, they learned how to endure the winter with fire, shelter and warm clothing.
Throughout the rest of the animal kingdom, from bears to bats to tortoises, extended periods of sluggishness are common. Not all are hibernators in the purest sense; in fact, some biologists view hibernation as a spectrum, with a state of temporary inactivity called “torpor” at one end. What's more, only one species in our own lineage — a diminutive primate called the fat-tailed dwarf lemur — is known to hibernate.
Can Humans Hibernate?
Though no human has entered hibernation, it’s already common practice to use hypothermia for medical purposes. In fact, therapeutic cooling isn’t even a modern idea — Hippocrates, a Greek physician known as the father of medicine, recommended in the 5th century B.C.E. that wounded soldiers be treated in the snow.
These days, doctors use it to limit damage to vital organs. Suppose you have a stroke or heart attack, in which blood (and the oxygen it carries) flows to the brain and heart are interrupted. Under normal conditions, that interruption will prove fatal within minutes. But when body temperature drops to hibernation levels, your tissues need less oxygen.
This is only a temporary kind of torpor. It’s less clear how we could safely hibernate for weeks, months or years on end. But a viable solution would go a long way toward unlocking the final frontier — deep-space exploration.
Is Human Hibernation Possible in Space?
One organization that’s particularly serious about human hibernation is NASA. At $10,000 per pound, the weight of food, supplies and infrastructure for a crew of astronauts could easily make distant voyages cost-prohibitive. But if these astronauts could snooze the flight away, they would drastically reduce their needs (and blissfully avoid many monotonous months).
Sound a little out there? As University of Texas biochemist Cheng Chi Lee writes in the Annual Review of Medicine, “the thought that humans can undergo a severe hypometabolic state analogous to hibernation borders on science fiction.” Indeed, it’s long been a staple of the genre, appearing in classic films like Alien and 2001: A Space Odyssey.
Yet since 2014, NASA has funded research on the prospect of long-term hibernation. The agency even developed a potential protocol for its use in future missions, once technology catches up to ambition.
With just one crew member awake and the rest in their pods for weeks at a stretch, the agency suggested, they’d be able to travel farther from home with fewer resources.
Could Drugs Induce Hibernation in Humans?
NASA's proposed endeavor comes with plenty of medical concerns, mostly revolving around our body’s stubborn attachment to a narrow temperature range — a few degrees off either direction and we find ourselves on the brink of disaster. Humans lack the adaptations that allow hibernators to lower their temperature without risk.
Experts point to the need for a drug, or some other method, that could overcome this physiological barrier. And although none have been tested on humans, animal trials have shown it’s possible to induce hibernation in species that don’t come by it naturally.
In 2011, a team of researchers at the University of Alaska Fairbanks applied their knowledge of Arctic ground squirrels (which do hibernate) to rats (which don’t). The cold-weather rodents get to sign off for the winter because they produce a molecule that stimulates their adenosine receptors, which are crucial to hibernation. Yet the team found that rats, when injected with a similar substance, doze off just the same.
More recently, researchers at Washington University placed rats in a state of torpor by firing ultrasound at the hypothalamus, a brain region that regulates sleep and body temperature.
Finally, an important distinction: Hibernation isn’t sleep. In fact, it seems to be so taxing that most mammals rouse themselves every few weeks throughout the winter to recuperate with actual sleep. So maybe it’s not the restful escape we imagine. Maybe you’re better off bundling up, gritting your teeth and embracing the eternal struggle to maintain 98.6 degrees.
Read More: Why and Where Snakes Hibernate