Could Humans Hibernate?

Some animals have it made. Their whole day revolves around eating and having sex (and, to be fair, trying not to get eaten themselves). And when winter arrives, they get to curl up somewhere and wait things out until the weather is nice again. Can humans get in on this hibernation thing?

First, a note on the lingo. While people often use the term loosely to refer to any state of dormancy in animals, true hibernation is a pretty specific thing characterized by “profound reductions in metabolism, oxygen consumption and heart rate.”

A hibernating animal’s core body temperature drops to extreme lows, sometimes matching the local outside temperature. As its body cools, its metabolism slows. This reduces the need for oxygen, and its breathing will also slow, sometimes to just one to five breaths per minute. The heart rate will likewise slow to just a few beats per minute. All of this ensures that the animal’s body will conserve as much energy as possible, which is necessary because it’s largely fueling itself with a limited supply of fat. "True hibernators" don’t shut down for the whole winter, though, and occasionally rouse to use the bathroom, eat from stored food, and stretch a little so their muscles don’t atrophy. Some animals may even switch hibernation spots.

Animals go into this energy-saving mode to ride out long stretches of environmental extremes, like lack of food and water, or very cold or very hot seasonal temperatures (dormancy during cold seasons is hibernation, and dormancy in the summer is called aestivation). Humans can deal with these situations while remaining active because we have things like canned food, greenhouse tomatoes, air conditioners, heaters, and turtleneck sweaters. Our bodies aren’t required to hibernate and we’re not perfectly adapted to it, but scientists have turned up a number of ways in which we’re pretty close.

Deep Sleepers

There are plenty of documented cases of humans going into hibernation-like states. In October, 2006, rescuers found Mitsutaka Uchikoshi 24 days after he’d gone missing on western Japan’s Mount Rokko. When they discovered him, he had no detectable pulse or respiration and his body temperature had dropped to 71 degrees Fahrenheit. Doctors would later confirm that his metabolism was almost at a standstill. When he woke up, remarkably showing no signs of brain damage or other ill effects, he explained that the last thing he remembered was falling on the trail and hitting his head. The entire time he’d been missing, he was unconscious, exposed to the elements and without food or water. The doctors who treated him said that the quick onset of hypothermia slowed his body down like hibernation would, and probably saved his life.

Similar survival stories include a Norwegian skier who’d fallen into icy water and woke up unfazed after showing no heartbeat, no respiration, and a core temperature of 57ºF, and the Canadian toddler who got lost outside on a cold night and was later revived after cooling to 61ºF and exhibiting no heartbeat for a full two hours.

In a controlled experiment in the early 1970s, the Yogi Satyamurti confined himself to a small, sealed underground pit in a state of deep meditation for eight straight days while being monitored by an electrocardiogram. At first, the yogi’s heart rate was normal, and then increased to 250 bpm for a while. On the evening to the second day, the ECG flatlined and remained like that until about 30 minutes before the pit was scheduled to be opened on the last day. The astonished researchers who’d been monitoring the yogi - whose core temperature had dropped four degrees in the pit - were sure that something was wrong with their equipment, but couldn’t find any malfunction or explanation besides the yogi’s heart stopping or decreasing in electrical activity below a recordable level.

It looks like our bodies have some of the abilities needed for hibernation. But like we said already, we’ve never had to, so our bodies aren’t completely adapted to the task. A few of the things holding us back are rather big obstacles. For example, researchers at the Paul Flechsig Institute for Brain Research in Leipzig discovered a few years ago that the brains of hibernating ground squirrels have brain cells containing modified proteins that are similar to those in the brains of Alzheimer's patients. The synapses that connect the brain’s neurons in both groups were also similarly degraded. The catch is that the squirrels’ brains bounce back after hibernation. They repair themselves and the animals show no signs of damage when they wake up in the spring, while human brains in the same state continue to deteriorate.

But hibernation on-demand would be useful to humans for reasons other than avoiding winter. Inducing hibernation in an accident victim on the way to the hospital could stave off extreme blood loss and cell breakdown, plus buy surgeons extra time to repair the injuries. It would also allow for the sort of space exploration that only seems possible in the movies. Placing astronauts in a dormant state, like in the Alien franchise, would allow them to sleep off the multiple years it would take for a spacecraft to travel to the outer reaches of the solar system and beyond.

Put to Bed

Researchers have been toying around with various ways to turn hibernation states “on” in animals over the last few years. Hydrogen sulfide seems to be one possible way to do that. By binding at the same cell sites as oxygen, the gaseous compound reduces the need for oxygen and depresses the metabolism. Mark Roth at the Fred Hutchinson Cancer Research Center in Seattle induced hibernation for the first time in lab mice in 2005 by having them inhale large doses of a hydrogen sulfide gas. Their metabolic processes slowed, their temperatures dropped, and then they snapped right out of it when they got a big breath of oxygen hours later.

Surgeons at Massachusetts General Hospital took a different approach in an experiment on Yorkshire pigs, to see how beneficial induced hibernation was in a trauma setting. After anesthetizing the pigs and giving them serious wounds that led to shock and extreme blood loss, the surgeons quickly chilled the pigs' bodies to 50ºF and pumped their veins full of a solution used for preserving transplant organs. At that point, the pigs were almost dead. They had little to no heartbeat, extremely reduced blood flow and no measurable electrical activity in the brain. The surgeons operated on the pigs and repaired their injuries. The pigs were revived when their temperatures were returned to normal and warm blood was pumped back in. The pigs bounced back with no noticeable physical or cognitive impairments.

While these are incredible breakthroughs and promising starts, we’re still a long way from making human hibernation simple, safe, and reliable. Other experiments failed to induce hibernation in sheep and pigs with hydrogen sulfide, so it might not work on larger animals, including us. Testing the Massachusetts method on humans, meanwhile, would be a bit tricky, ethically speaking. It’s a start, though, and sooner or later we might move beyond mere sleep and hibernate our way through surgery, or a flight to Jupiter.