Do Bears Really Hibernate?

Last updated: October 11, 2026

Yes and no. Bears do spend months tucked away in dens without eating. But a bear in winter isn't in the deep, near-frozen sleep that people often imagine, and a few kinds of bears don't do it at all.

A different kind of winter sleep

Small hibernators like ground squirrels let their bodies cool down to near freezing, and it takes them a long time to wake up.

Bears work differently. During their winter rest, a black bear's body temperature drops only a little. Its metabolism, though, slows to about a quarter of its normal rate. That combination lets a bear save a huge amount of energy while staying warm enough to wake up fairly quickly.

Because of this, you'll see it described in different ways. Some park biologists call it a shallow torpor rather than true hibernation, while other researchers call bears hibernators because of how long they go without food or water. The name matters less than what's going on inside the bear.

What happens inside a resting bear

• The heart slows way down. In a study of black bears in Alaska, heart rates dropped from about 55 beats per minute in summer to about 14 in the middle of winter, sometimes with up to 20 seconds between beats.
• No eating, drinking or bathroom breaks. For months on end, black bears can go without eating, drinking, peeing or pooping.
• Fat is the fuel. A denning black bear still burns about 4,000 calories a day, all from the fat it stored up in the fall.
• Muscles stay strong. Unlike a person stuck in bed for months, bears keep most of their muscle. Researchers report they lose only about 10 to 20 percent of their strength over a winter.

Can a bear wake up in winter?

Yes, and fairly quickly. Black bears in their winter state can wake up if they're disturbed. That's one reason to never go near a bear den, in any season.

I can wake up. You of all people should know that.

Fall: the big eat

All that winter fasting needs fuel, so bears spend the fall eating almost nonstop. This feeding frenzy is called hyperphagia. Wildlife agencies estimate a black bear can eat up to about 20,000 calories a day and spend up to 20 hours a day looking for food.

That's also when bears are most likely to get into trash cans and bird feeders, so fall is a good time to lock up anything that smells like food.

Where bears spend the winter

Not always in caves. Black bears often use whatever shelter they can find: tree cavities, burrows, rock crevices or the space under a fallen tree. Grizzlies usually dig their own dens.

How long they stay depends on where they live and on the bear. In the Greater Yellowstone area, female grizzlies without cubs den for about five months on average. In colder places, black bears can spend up to about seven months in their dens.

Babies born in the den

Here's one of the strangest parts. Black bears mate in summer, but the pregnancy pauses: the fertilized egg only attaches and starts to grow later in the year, around the time the mother settles into her den. This is called delayed implantation.

The cubs are born in the den in the middle of winter, tiny, blind and toothless. They nurse while their mother rests, and the family comes out in spring, when fresh green plants appear.

Bears that skip winter

Not every bear takes a winter break.

• Sun bears and sloth bears live in warm places where food is available all year, so they don't hibernate.
• Giant pandas don't hibernate either. Bamboo is around all year, and they spend most of their day eating it.
• Polar bears mostly stay active and keep hunting through the Arctic winter. Only pregnant females settle into snow dens to give birth. Snow covers the entrance, and the mother's body heat can keep the den up to about 45°F (25°C) warmer than the air outside.

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