How Do Animals Sleep?

Animal sleep habits are just about as diverse as animals themselves. A variety of factors help determine how long animals sleep, where they rest, and how much time is spent in different sleep stages. Age, species, diet, size, and environment all come into play, and there are some sleep habits that scientists simply haven’t figured out yet.

Sleep seems elemental, but similar to with humans, experts don’t fully understand why animals sleep. Spending time in this vulnerable state can be dangerous, and it takes time away from reproducing or hunting for food. Sleeping must be important, then, or animals probably wouldn’t bother to do it. There’s evidence to show lack of sleep can negatively impact animals, too. 

Learning about different animals’ sleep habits is interesting in and of itself, and scientists also hope that sleep research on animals might help shine light on why we humans sleep.

From the armadillos who spend the majority of each day asleep, to the giraffes who receive just a few hours, most scientists believe that virtually all animals sleep. Some experts are reluctant to say definitively that certain animals, like reptiles and invertebrates, actually sleep, but a lot of that hesitance has to do with a lack of research on specific species. That said, current research points to the theory that animals all rest in some way. 

Sleep looks different depending on the animal, but it’s usually defined as a period of being relatively still, with reduced reactions to the outside world. Looking at certain groups of animals shows how sleep habits can vary.

Land Mammals

Most land mammals sleep for some period of time during the day, and there’s some evidence to suggest many even dream. Mammals’ sleep needs vary depending on factors like size, age, where they live, and whether they’re predator or prey. Animals that hibernate, like bears, tend to sleep for different lengths of time depending on the climate and season.

Predators typically sleep longer than prey, especially if they have no predators of their own. Animals that are lower on the food chain tend to only sleep long hours if they’re able to find a protected sleeping space. The largest land mammal, the wild elephant, sleeps just two to three hours a day in total, mostly in the form of short naps while standing up. 

Smaller omnivores like armadillos may sleep up to 20 hours a day, possibly because they find it easier to hide while sleeping. Three-toed sloths have also been reported to sleep around 16 hours in captivity. When nocturnal predators are nearby, sloths switch to sleeping predominantly at night to avoid detection. Other mammals show a similar tendency to change sleeping habits depending on the presence of predators.

Marine Mammals

Dolphins and whales display what’s known as unihemispheric sleep, which means that half the brain remains awake. During unihemispheric sleep, marine mammals may be less responsive, but they keep swimming and can still monitor their environment to some extent. It’s thought that unihemispheric sleep allows marine mammals to keep surfacing for air and regulate their temperatures more easily. 

Sleep is a little different for mammals that live on both land and in water. Walruses can swim for an impressive 84 hours straight, but they prefer to do their sleeping on land. By contrast, fur seals prefer to sleep in the water, where they engage in unihemispheric sleep. 

Researchers have failed to find examples of REM sleep in many marine mammals. REM, or rapid eye movement, sleep is when dreaming and memory consolidation happens. It’s usually accompanied by a lack of muscle movement, which could be dangerous when in the water. That said, animals like fur seals and walruses display periods of REM sleep when they choose to sleep on land.

Reptiles and Amphibians

There is little research on sleep in reptiles and amphibians, and it was once thought many of them didn’t sleep at all. The bullfrog, for example, gained a reputation for being an animal that never sleeps, based on research from the 1960s. 

However, scientists have since broadened their definition of what sleep can look like in animals. Newer research on iguanas and bearded dragons suggests that reptiles do experience a sleep-like state. Certain reptiles, like crocodiles, engage in unihemispheric sleep, like marine mammals. And while there’s less data on sleep amphibians than reptiles, we do now know that they experience some form of a rest phase.

Birds

Scientists originally believed that birds were able to go for many days without sleeping because many species can fly for days without stopping. However, scientists now believe that birds might actually be able to sleep while they fly.

Like many animals, ducks tend to sleep in groups for better protection. Among ducks sleeping in a row, those on either end sleep with one eye open to watch for predators, while ducks in the more sheltered positions sleep with both eyes closed.

One study found that birds showed more REM and deep sleep when they were allowed to sleep on a higher perch, farther from predators.

Fish

Scientists believe several types of fish show clear distinctions between being asleep and awake. Some of these fish become completely still, while others keep their fins moving in order to breathe. Sharks and rays also show periods of reduced activity, but it’s unclear whether these larger fish are sleeping or simply resting.

New research has also found that jellyfish appear to sleep. This is important because jellyfish are one of the oldest known life forms still around today. If jellyfish sleep, it could support the theory that sleep played an important role in evolution.

Invertebrates

We tend to think of invertebrates as very different from ourselves, but it’s been shown that they need sleep, too. Flies and crayfish display many of the same brain activity patterns as humans do while they sleep. Flies and cockroaches die if they go too long without sleeping. A very rudimentary type of flatworm appears to sleep on a daily basis, reinforcing the idea that sleep evolved in primitive life forms.

Interestingly, bees seem to sleep different amounts of time depending on their age and their role within the bee social structure.

As humans, we typically feel our best when we’ve had a good seven to nine hours of shut-eye. What feels like a reasonable amount to us is excessive to much of the animal kingdom. Sure, lions and koalas can snooze for hours on end, but many animals sleep very little. In fact, some animals sleep so little that they would consider your afternoon power nap to be a full night’s sleep.

There are exceptions to every rule, but in general, large animals tend to sleep fewer hours than smaller ones. Some grazing animals also get by on less sleep than their hunting counterparts since they need to spend so much of their day eating to survive. Let’s look at some of nature’s most successful power nappers.

Sheep

Sheep are diurnal like humans, meaning much of their sleep takes place at night. In total, they may only sleep for about five hours in a day. Ruminant animals like sheep must spend much of their day upright in order to eat, which limits their ability to sleep lying down. As a result, they may spend as little as 2.5% of their sleep in REM, a stage of sleep that can typically only happen when an animal is lying down. By comparison, humans spend up to 10 times that amount in REM sleep.

REM sleep is also shorter in prey animals like sheep, since spending too much time in that stage of sleep can make them more vulnerable to attack. Sleeping in a herd offers them some protection against predators.

Giraffes

The tallest animal in the world gets by on very little sleep. In total, a giraffe sleeps around four and a half hours per day. As grazers, giraffes spend most of their day eating. Much of their sleep takes place in short naps lasting 35 minutes or less. 

Elephants

As grazing animals, elephants have short sleep needs. In all, the average elephant may only sleep two hours per day. That two-hour figure is only an average, and some elephants have been observed traveling for nearly two full days without sleep to escape poachers. Due to their grazing and itinerant behavior, elephants choose new sleep spots each night. 

Walruses

Walruses can spend up to 84 hours swimming continuously. Given that they can stay awake for more than three days, they certainly deserve a spot on our list. When they finally get a chance to rest, walruses may sleep while floating in the water, lying along the sea bottom, or leaning against something in an upright position.

Walruses can sleep on both water and on land, but they prefer to snooze on shore, and will spend up to 75% of their time on land asleep. In total, they might spend anywhere from 2 to 19 hours resting, and sleep in short bursts of 3 to 23 minutes.

Migrating Birds

When considering which animal sleeps the least, it looks like migrating birds may be the winner. These animals are the ultimate multitaskers, having figured out a way to sleep while they fly. Similar to marine mammals, birds have the ability to sleep unihemispherically. This enables them to keep one eye open and maintain flight while migrating.

Alpine swifts have been documented flying for 200 days nonstop, while frigate birds sleep for less than one hour per day during a 10-day flight. When they reach land, these birds catch up on sleep just like walruses do, and may sleep for up to 13 hours per day.

Another way that animals’ sleep habits differ from ours is that certain ones have evolved to sleep while standing up. Researchers hypothesize that sleeping in an upright position may make it easier for them to wake up and get away if a predator approaches during sleep.

The bodies of animals that can sleep while standing have developed a way to keep them upright while asleep, usually by locking a set of muscles and tendons into place. However, studies show most of these animals can only stay upright during non-REM sleep. During REM sleep, the body experiences a temporary loss of muscle tone, so most animals need to lie down for REM sleep.

The exception to this rule is birds. Researchers have found that certain species of birds seemingly enter REM sleep while standing. These birds might show a loss of muscle tone in their necks as they sleep and stand, but they still maintain enough muscle tone in one leg to balance and stay upright. Scientists are not quite sure how they do it.

Horses

Studies show that horses spend five to seven hours of their day asleep or resting, most of it at night. Around 80% of their sleep takes place in an upright position. They must lie down for at least 30 minutes of their daily sleep in order to obtain just under five minutes of REM sleep.

Experts have learned that horses sleep standing up by using a “stay apparatus.” Essentially, one hind leg muscle locks the knee into place, enabling the leg to stand upright and support most of the horse’s body weight without requiring much energy at all. Meanwhile, the other hind leg takes a break, bending slightly with the hoof grazing the ground.

Elephants

Elephants can sleep standing up or lying down, but, like horses, they can’t enter REM sleep while standing. In fact, they may only enter REM sleep every few days. Scientists have found a few indicators to determine when an elephant is sleeping. When standing up, elephants close their eyes and let the end of their trunk droop to the ground. If an elephant hasn’t moved for five minutes, it’s safe to assume it’s asleep.

Giraffes

Giraffes can also sleep while standing up, head up and eyes sometimes open. To enter REM sleep, they lie down, their eyes close, and their head falls to the side as their muscles relax. Like elephants, giraffes are herbivores. Herbivores typically sleep less than carnivores, and the larger the herbivore, the less they tend to sleep.

Flamingos

Like horses, flamingos can sleep while standing thanks to a stay apparatus. This anatomical arrangement enables the muscles and ligaments in one leg to lock into place and keep them upright without exerting much effort. Flamingos often stand on one leg while awake and asleep, using the stay apparatus to save energy. You can tell when they’re sleeping when their eyes close and they sway less.

Cows

On average, dairy cows sleep for four hours each day. This sleep occurs in short bursts of three to five minutes. In total, cows spend about three hours in non-REM sleep, and 30 to 45 minutes in REM sleep per day. They stand up while in non-REM sleep, but like other large mammals, must lie down to enter REM.

Cows spend about half their day lying down, but they’re only asleep for some of that time. Like sleep, lying down appears to serve an important biological function for cows. If they’re prevented from getting their lying time, they’ll act out by stomping around or even headbutting other cows.