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.

Some Animals Don’t Actually Sleep for the Winter, and other Surprises about Hibernation
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.
adenosine
Noun

one of the four molecular building blocks of DNA.

body temperature
Noun

heat energy radiated by a person or other animal. Also called normothermia or euthermia. For humans, resting body temperature is 37 degrees Celsius or 98.6 degrees Fahrenheit.

comatose
Adjective

being in a coma-like condition, which is a state of deep unconsciousness ( unrelated to sleep).

hibernation
Noun

state of reduced physiological activity, similar to sleep, in which some animals spend the winter.

metabolism
Noun

chemical changes in living cells by which energy is provided for vital processes.

physiology
Noun

study of activity in living organisms, including physical and chemical processes.

Noun

group of similar organisms that can reproduce with each other.

torpor
Noun

state of reduced physiological activity similar to hibernation, usually evidenced by lower metabolic rate and body temperature. 

unconscious
Noun

unaware, asleep, or in a sleep-like state.