Some Animals Don’t Actually Sleep for the Winter, and Other Surprises About Hibernation

Some Animals Don’t Actually Sleep for the Winter, and Other Surprises About Hibernation

It isn’t just groundhogs—find out which animals hibernate and why.


3 - 12


Biology, Ecology, Storytelling


Arctic Ground Squirrel Upright

Hibernating animals slow their metabolisms, cooling their bodies by 5° to 10°C (9° to 18°F). Arctic ground squirrels (Spermophilus parryii) can take this much further, cooling their bodies to subfreezing temperatures.

Photograph by Thomas and Pat Leeson
Hibernating animals slow their metabolisms, cooling their bodies by 5° to 10°C (9° to 18°F). Arctic ground squirrels (Spermophilus parryii) can take this much further, cooling their bodies to subfreezing temperatures.
Selected text level

For people who aren't fans of winter, animals that hibernate seem to have the right idea. After all, hibernation is like burying your head under the covers until spring comes, isn't it? Not quite. The science of hibernation is very different from what happens when you sleep.

What Is Hibernation?

Hibernation is a long-lasting form of torpor. This is a state where metabolism is highly reduced. Metabolism is the chemical process that takes place in plants and animals to keep them alive. It is how our cells turn the food we eat into energy. During hibernation, metabolism is "extremely slowed down or completely halted," scientist Marina Blanco said. She studies the dwarf lemurs (Cheirogaleus spp.) of Madagascar. They are the only primates that hibernate on a regular schedule.

When dwarf lemurs hibernate, they reduce their heart rates. An active lemur's heart can beat up to 300 times a minute, Blanco said. During hibernation, it can beat less than six times a minute. Breathing slows down, too. Instead of taking a breath every second, hibernating lemurs can go up to 10 minutes without taking a breath. Their brain activity "becomes undetectable."

This is very different from sleep, which is a gentle resting state. During sleep, unconscious functions like breathing are still carried out. Hibernation is a much deeper kind of rest. In fact, hibernators sometimes "wake up" from their hibernation in order to catch some sleep.

Why Do Animals Hibernate?

Kelly Drew is a scientist who studies hibernating Arctic ground squirrels (Spermophilus parryii). She said that hibernation is a way for animals to save energy.

Hibernation is usually associated with cold winters. However, animals that live in cold climates aren't the only ones that hibernate. There are tropical hibernators that might do it to beat the heat.

Sometimes, hibernation is not based on outside temperature. "Some species hibernate in response to food shortages," Drew said. For example, a group of spiny mammals in Australia called echidnas will hibernate after fires. They wait until food grows back before they go back to normal activities.

Recent studies have even suggested that hibernation might be a form of protection. Thomas Ruf is one of the scientists who believe this. When hibernating, "you don't smell, you don't make any noise, you don't make any movements," he said. That makes it hard for predators to find you.

What Actually Happens when Animals Hibernate?

To slow their metabolism, animals cool their bodies. On average, they reduce their temperature by 5 to 19 degrees Celsius (9 to 18 degrees Fahrenheit). However, animals don't stay in their cold, frozen state the entire time. Sometimes they wake and warm up.

Ruf said it's a great mystery why they do this. Some scientists think the animals need to turn their immune systems back on to fight disease. Others think they may simply stop hibernating so they can sleep.

What Kinds of Animals Hibernate?

There are many mammals that hibernate. Bears might be the first that come to mind. However, there have been questions for years about whether bears are really hibernators. Unlike animals that stir regularly during hibernation, bears can go for about 100 days without having to wake up to eat. They are also easier to wake up than typical hibernators.

Most mammals who hibernate are on the smaller side. The average hibernator weighs less than 91 kilograms (one-fifth of a pound), Ruf said. That's because small animals tend to lose heat more quickly. They need to hibernate to save energy more than larger animals do.

What Animal Hibernates the Longest?

It's hard to say which animal can hibernate the longest. Edible dormice (Glis glis) could be a good choice. These tiny rodents can hibernate for more than 11 months at a time in the wild.

However, in one experiment, a big brown bat (Eptesicus fuscus) hibernated in a refrigerator for 344 days. Maybe that makes bats the winners. Yet, the bat in this experiment didn't exactly choose to hibernate on its own. It also didn't survive its long hibernation.

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Tyson Brown, National Geographic Society
Heather Brady
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Clint Parks
Last Updated

February 26, 2024

For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.


If a media asset is downloadable, a download button appears in the corner of the media viewer. If no button appears, you cannot download or save the media.


Text on this page is printable and can be used according to our Terms of Service.


Any interactives on this page can only be played while you are visiting our website. You cannot download interactives.

Related Resources