During the winter, time slows down. You may find yourself curling up with your Netflix for hours, sleeping for half the day, or staying indoors for unhealthy periods of time. While doing so, you may wonder: why not just go all out, skip the unpleasantness and spend the whole winter unconscious in a warm, cozy burrow?
Researchers who hope to prepare humans for extended space travel have the same thought. They want to know: Why don’t humans hibernate—and could they in the future?
To answer that question, first we have to understand exactly what hibernation is. Here's what we know: Hibernation is when an animal goes into a state of extended inactivity. Often hiding itself away from the elements and predators, the animal's metabolism will slow to less than a quarter of its normal rate. This cools the creature's body and slows its heartbeat to just a few times per minute, for up to months at a time.
"They turn down the pilot light really," says Kelly Drew, a neuropharmacologist at the University of Alaska Fairbanks who studies the brains of hibernating Arctic ground squirrels. Hibernation is often stimulated by harsh conditions, shorter days or colder temperatures, which drive an animal to begin putting on fat and otherwise preparing to hibernate. The creature will then stay in this period of inactivity until it gets the right signal—light or temperature—to stir from its torpor.
Yet many misconceptions surround this powerful adaptation. For one, although hibernation resembles a deep sleep, it is not thought to be related to sleep at all. In fact, it may actually leave animals with a sleep-debt after they awaken from it, Drew says. "Hibernation is really energy conservation," she says. "It's an adaptation to resource limitation." Much like migration for birds, hibernation evolved to allow animals to make it through periods of extreme hardship.
"Some animals leave, some animals hibernate, and some animals just live with it," Drew says. (This explains why animals in zoos don't hibernate: A creature with plentiful food, water and shelter will have no need to undergo the process.)
For another, we usually think of hibernation as being associated with winter, which is when animals like bears, squirrels and groundhogs will check out for several months. But hibernation is not exclusively a cold-weather behavior, Drew says. Reptiles, amphibians insects and even a few mammals, such as the Madagascan fat-tailed dwarf lemur, practice the warm-weather hibernation—known as aestivation—to avoid periods of drought or extreme temperatures by chilling out underground.
Yet while researchers understand how hibernation could have evolved, surprisingly little is know about the actual process happens within the body. So far, scientists haven't found any unique genes in hibernating animals, for example. Furthermore, nobody knows what exactly is regulating those existing genes to allow the process of hibernation in some animals and not in others—like humans, for instance.
“We don’t even know which gene expression is necessary to hibernate,” Drew says.
This mystery has just become more urgent as humans attempt the first manned mission to Mars. The idea of “deep sleep”—once the purview of deep space sci-fi films like Alien and now Passengers—is now a real possibility for scientists who want to ensure that human astronauts can survive during a months- or even yearslong journeys to other planets. One of those scientists is Drew, who is consulting with a company called Spaceworks Enterprises on a NASA-funded project to put humans into hibernation for spaceflight.
There are no known cases of natural human hibernation, according to Drew. But she has heard anecdotes about hibernation-like experiences in her research, including the practice of "lotska," in which Russian peasants a century ago would supposedly endure the harsh winter by awaking only once per day for 6 months to consume a small amount of bread and ale. Harvard University cardiologist Herbert Benson has also chronicled Buddhist monks who were able to lower their metabolism by 64 percent through certain meditation techniques.
Currently, however, Spaceworks is looking into techniques already being used in medicine, namely therapeutic hypothermia. First used medically more than 70 years ago, this technique involves cooling a person's body to near the freezing point of water (32 degrees F) to slow their cellular and brain functions. This technique can protect a patient's tissues from damage due to lack of oxygen or blood, usually after a heart attack or surgery such as a heart transplant. (Note: this should not be confused with the trend of cryotherapy, which has little scientific backing and has been linked to several deaths.)
While therapeutic hypothermia is usually used for periods of a day or two, Spaceworks president John Bradford notes that a woman in China was kept chilled for two weeks to allow her brain to heal from a traumatic injury. She recovered fully and returned to her daily life. But no other use of therapeutic hypothermia at that length or longer has been recorded, so it remains unclear how long the technique can be used safely.
Bradford envisions putting people into weeks-long increments of hibernation using body and brain-cooling techniques similar to therapeutic hypothermia to conserve space and resources for large-scale trips to Mars. "It's closer to reality than it sounds, but there's still a lot of questions, and a lot of development that needs to occur," Bradford told Popular Science about the project in December.
Yet there are still several hurdles to overcome, Drew points out. Hibernation isn’t simply a matter of turning the knob on your metabolism; it involves a host of other related adapations. Foremost among these is waste management. Animals that hibernate are able to essentially halt their urination and defecation during hibernation, Drew says, sometimes through a process of reabsorption to preserve nutrients. Unfortunately, humans can’t do this, though Drew has heard of proposals such as using rectal catheters.
And even if we figure out the poo problem, there are other challenges. Body temperatures below 37 degree Fahrenheit tend to disrupt the human digestive tract and may cause pain. Cold temperatures can also suppress the immune system, making people more vulnerable to infections. It may turn out that humans simply weren’t meant for hibernation.
On the less scientific side, the human in question would have to agree to hibernation—which may not be an appealing option. Putting oneself into a vulnerable state of hibernation and missing out on weeks or months of your life is something many people might be hesitant to choose. Bradford recounted to Inverse magazine that astronaut Buzz Aldrin objected at a conference presentation to the idea of hibernating, because he wouldn't want to miss out on the journey there.
However, Bradford thinks that practical concerns will eventually win out. “If it means the difference between being able to go to Mars, and not,” says Bradford, “I think everybody will do it.”
If scientists can make it possible for humans to hibernate during spaceflight, it will certainly be a great achievement. But perhaps the most amazing thing about hibernation is that humans have learned to survive without it—in environments as harsh as the Arctic and the tundra—for millennia. From adapting our diets to building igloos to wearing animal skins, Arctic communities have more than compensated for our inability to enter a winter-long torpor. So while we may marvel at bears and doormice, give us some credit, too.