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Do Hibernating Animals Really Sleep All Winter?

Eastern Chipmunk hibernating (Tamias striatus)
Breck P. Kent/Shutterstock

If you decide to stay in bed on a cold winter morning, you might feel like you have something in common with hibernating animals. However, hibernation is actually a lot different from sleep—and a lot more interesting.

Many kinds of animals hibernate, from bears to lemurs to snails. But hibernating doesn’t just mean finding a safe place to relax while winter storms play out. Instead, it’s a fascinating physiological process that humans can’t do.

So, what is hibernation, and why do animals do it?

How Hibernation Works

When an animal hibernates, it actually drops its metabolism to below five percent of the normal rate. This means its heart rate drops, its blood flow decreases, and it stops needing food and water. Hibernating animals don’t even use the bathroom while they hibernate. Their body temperatures also tend to drop far below the norm.

This physical state is known as torpor—but hibernation is an extended version of this state. If a human entered a state like this, it would be a sign of serious medical problems. But for many animals, torpor and hibernation are important survival skills.

Why Some Animals Hibernate

When the weather gets cold, producing body heat takes an extreme amount of energy. For many animals, it’s not possible to find enough food during the winter to sustain that kind of energy. Entering hibernation, with its drop in body temperature, enables animals to conserve energy so they can live through the winter months.

However, hibernation doesn’t always mean staying in one place for the whole winter. For example, bears often shift positions and even move around during their hibernations. And, if the den gets damaged in some way, a hibernating bear will leave in search of another one.

Some animals also enter torpor periodically without going into full-on hibernation. Many birds normally spend part of the day in torpor, for example. But hibernating animals spend weeks or months at a time in this state, which is why they prepare by storing up extra body fat or stashing food that will keep them going through the winter.

Scientists actually disagree on exactly what counts as hibernation. For example, because bears move around so much during their hibernations and don’t drop their body temperatures significantly, some argue that bears don’t truly hibernate. However, others say that because the bears’ metabolisms still drop a lot during this stage, it does count as hibernation.

Most of the animals that hibernate (by the strict definition) are much smaller than bears, such as rodents and bats. Larger animals often migrate for the winter, but small animals can’t travel far enough to migrate, so hibernation becomes a valuable survival tactic for them.

Hibernation also isn’t just limited to cold places. Some animals in warm or tropical climates also hibernate. But again, it’s about saving energy, not staying warm: these animals often hibernate during the drought season when food and water become hard to find.

What Causes Hibernation?

Because animals hibernate at different times and for different reasons, how do they know when to enter this state of long-term torpor?

For animals in cold-weather regions, a drop in temperature might be the trigger. Shorter days and shrinking food supplies can also help an animal know when to start hibernating.

Yet, in artificial environments without shorter days and cooling temps, many animals still enter hibernation. This suggests that there’s some instinct beyond environmental triggers that tells them when to begin.

A brain chemical called adenosine appears to regulate the animal’s body temperature during this time, cooling it to the point of hibernation. However, animals will “wake up” periodically during hibernation before entering it again for reasons scientists also don’t fully understand.

Like many aspects of animal behavior, hibernation is hard to study, because we can’t completely replicate the conditions that cause it in a lab environment. There are still many questions about it that science has yet to answer. However, one thing’s for sure: hibernation and sleep are two very different things.

Elyse Hauser Elyse Hauser
Elyse Hauser is a Seattle-based writer and editor with a Master's in Writing Studies from Saint Joseph's University. Her work has appeared in publications like Racked, Vine Leaves Literary Journal, and Rum Punch Press. She was awarded a 2017 Writing Between the Vines residency.  Read Full Bio »