Nighty Night, Sleep Tight! Hibernation, Brumation and Torpor Explained

Updated 9/24/2026

Rest and Relaxation. Catching some ZZZ’s. Hibernation, Brumation, and Torpor are all animal responses to dealing with the need to conserve energy and slow down. I explain what they are and how wildlife gets through the cold months every year.

Hibernation, Brumation, and Torpor are biological reactions to the environment. By slowing down their metabolic rate animals can conserve energy, have less need for food, and gain the ability to survive lower temperatures.

Grizzly bear fishing for salmon.

Producing Energy

Animals use energy all day. Things like walking, digestion, heating and cooling their bodies, all of these things use energy. Even thinking uses energy.

Energy is produced when animals eat. Just like putting gas in a car to make it go. However, food sources aren't necessarily available all year long.

Animals slow down their body’s need to produce energy and they become inactive. But their level of inactivity goes much deeper than you slumping out on the couch binge-watching Grey’s Anatomy.

Grab a warm blanket and curl up by the wood stove. Let’s break it all down and look at each one.

Hibernation

Many of us think of bears when we hear the word hibernation. but scientists now say that bears experience torpor.

Hibernation is a state of dormancy where an animal's metabolic rate, heart rate, and body temperature drop significantly, conserving energy and reducing the need for food. During this period, animals rely on stored fat reserves accumulated during the warmer months.

Unlike regular sleep, hibernation involves dramatic physiological changes. In mammals, body temperature may drop to near freezing, and heart rates slow to just a few beats per minute. Hibernation is a voluntary state that animals enter based on environmental factors like shorter days.

Depending on the animal, and the climate they may hibernate for weeks or months. This depends on the severity of the climate and the local food supply.

Fun Fact: Hibernation is the winter term for lowering metabolism. The opposite, aestivation, is what animals do in summer when they slow down their body functions.  

Coloring books are available on Amazon in paperback and also as a digital download in our website store. Purchases help to support wildlife rehabilitators.

Heterothermy

Heterothermy is the term scientists use to describe an animal’s ability to lower its body temperature to match the surrounding air temperature. This is an active metabolic response which means the animal can control when it happens.

So a bear will receive clues from the surrounding environment and have the urge to store up fat (eat lots) and find an area of the shelter.

Learn more about grizzlies: The Epic Grizzly and What We Learn From Grizzly 399

True Hibernators

For an animal to be considered a true hibernator, its body must undergo three main physiological changes to conserve energy:

• Lowered body temperature

• Slower Heart Rate and

• Reduced metabolism

Groundhogs/Woodchucks

Groundhogs are true hibernators, dropping their body temperature from a normal 99°F (37°C) down to as low as 37°F ( 3°C) during their winter sleep. Their heart rate drops dramatically from a normal 80 beats per minute down to just 5 beats per minute.

Even though they are true hibernators whose heart rates and body temperatures drop drastically, they do not sleep continuously for months without interruption. Depending on the species and the climate they may hibernate for weeks or months.

Some animals like the groundhog, cycle through sleep and wakefulness.

Read The Dynamic Digger - The Groundhog

Artic Ground Squirrel

Arctic ground squirrels are the champion sleepers and spend 7 to 8 months in hibernation. Researchers at the University of Alaska at Fairbanks have shown that during hibernation, arctic ground squirrels adopt the lowest body temperature ever measured in a mammal. 27 F (−3 °C). They wake up every few weeks and spend a couple of days warming their bodies before going back to sleep.

Hibernation is triggered by decreasing day length and hormonal changes.

Cave myotis bat in Oklahoma

Bats

Many bat species around the USA eat insects. In the winter insects are not an available food source so bats either migrate south or hibernate during cold months.

Bats that hibernate include the Little and Big Brown bat, Eastern Pipistrelle, and the Northern long eared bat.

A bat's heart rate drops from hundreds of beats per minute while flying to as low as 20 beats per minute. Their body temperature drops close to freezing and they slow down their breathing rate. Hibernating bats rely on stored body fat, and lose up to a quarter their weight.

Bats gather in safe, cool underground locations called hibernacula, such as caves and abandoned mines. Some species choose rock crevices, hollow trees, or human spaces like attics and crawl spaces.

Some bats like hoary bats migrate south for winter when insects become scarce.

Read my article: Kentucky Bats

This video is all about hibernation, brumation, and torpor. What they are and how does wildlife get through the cold months every year. Dormancy is the biological reactions to the environment. By slowing down their metabolic rate animals can conserve energy, have less need for food, and survive lower temperatures.

Hormones

Hibernation is not as easy as just going off to a nice warm cave. It is directed by the endocrine system. Glands in the body regulate hormones that are released at key times. They control the physiological reactions of hibernation.

• The thyroid gland controls metabolism and activity levels.

• The pineal gland secretes melatonin the hormone that controls growing winter coats

• Pituitary gland that regulates metabolic functions, storing fat, heart, and breathing rate

• The pancreas produces insulin the hormone that regulates glucose and energy levels

Some animals hibernate through gestation and give birth. I wish I had known about that when I had my children!

There are actually several different types of hibernation.

Obligate Hibernation

These are animals that hibernate seasonally. Ground squirrels hibernate every winter and are obligate hibernators.

Facultative hibernation

Some animals hibernate in response to environmental stressors such as lack of food or draught. This may often overlap with what we call torpor because these animals may wake up during winter and go look for food.

Difference Between Sleep and Hibernation

Hibernation is much more than sleep. In fact, animals who come out of hibernation may be tired and cranky and need more sleep.

In hibernation, the animal undergoes physiological changes that can be quite dramatic. Temperatures may drop sixty degrees. A hibernating animal does not awaken easily and loses consciousness. Waking up a hibernating animal may actually harm it and cause death.

Sleeping is a period of rest that has a slower heart and breathing rate but nothing drastic. The brain is quite busy when we sleep. Not only does it continue to give commands to our bodies such as “repair these tissues” and “we need to grow” it is also very active dreaming.

Torpor

Instead of storing fat in their bodies, animals like squirrels and chipmunks store food. They generally sleep for short periods, waking up every few days to eat from stored food rather than staying deeply asleep all winter.

Torpor can be a confusing term because it can refer to a short or long period and can happen at any time of year. Torpor is a period of “decreased physiological activity” such as reducing their body temperature and metabolism.

Sometimes torpor occurs at night after a day of looking for food. This nighttime rest allows the animal to conserve energy and deal with the colder weather.

Torpor has been observed in hummingbirds as they migrate. The hummingbird needs immense amounts of energy to reach the southern climates. They depend on the fat they have stored from summer pollen. During migration, the hummingbird flies during the day and enters torpor during the night. This allows them to conserve energy needed for flight.

Fun fact. Torpor has a survival advantage. Mammals are less likely to go extinct if they utilize torpor.

Torpor is not just for winter. The Rufous hummingbird who lives up in the Rocky mountains may enter torpor every night because even in summer temps can go below freezing at higher elevations.

There are several species of chickadees that overwinter. These high energy little birds are fun to watch during the cold winter months. They are also a good example of a bird that practices torpor.

Read my article: How Do Birds Survive In The Winter

Chickadees feed on dormant insects, berries and cone seeds of evergreens during the cold winter months. A study in the Journal of Comparative Physiology showed that the Black-capped Chickadee, Parus atricapillus, was able to lower their body temperature by about ten degrees below normal. This saved them up to twenty-three percent of their metabolic expenditure.

Another bird that uses torpor but does not migrate is the common poorwill, a nocturnal bird that survives on insects and lives throughout western America. During winter they can sleep or enter torpor up to five months.

The common poorwill can lower its body heat to 41° Fahrenheit. That pretty low considering humans run at 98.6 pretty consistently. So when there food source is scarce they go into torpor. Pretty clever really.

Brumation

Reptiles brumate. The difference in brumation and hibernation is simple. Reptiles can not control their metabolic response. As an ectotherm, their body temperature is based on the environment.

When the nights get colder and the days shorter it lets our reptile friends know that it is time to get underground. Turtles and snakes typically go underground where the temperatures are more stable. Our friends the box turtles typically go down about twelve inches.

Reptiles often brumate together. In fact, some cohabitate. In the south gopher tortoises dig holes to brumate and are often joined by the Eastern diamondback rattler who cant dig very well.

Gardner snakes often brumate in groups. On warm days they may come out to sun themselves and get a drink. Read our article on Kentucky snakes.

Painted turtles

There is a pond near me full of these guys. Painted turtles. I love watching then come out and sun themselves in spring. but during the winter the pond freezes over and the turtles go into brumation.

Brumation allows painted turtles to conserve energy and reduce their need for oxygen. They dig down in the mud at the bottom of the pond. They can survive without food or much oxygen for up to 100 days.

During brumation, turtles don't require much oxygen but what they do get comes in there butt! The turtles rectum or cloacal to be specific is the part of their body where they expel waste. Also where eggs come out on females. It has a lot of blood vessels that can absorb oxygen from the water.

The scientific term for butt-breathing is cloacal respiration.

Author, Ame Vanorio has 29+ years of experience living off-grid and as an organic farmer. She is the director of Fox Run Environmental Education Center and a licensed wildlife rehabilitator. Ame is an author and you can check out her books on Amazon.