Are There Animals That Don't Sleep?

Before meeting the animals that sleep the least, it's important that we answer the question the title of this article poses: are there animals that don't sleep? While it may seem like some animals don't need rest, there are none that don't need to restore their energy through periods of inactivity. Whether this means they don't sleep is a question of definition.

Sleep can generally be defined as a period of relative inactivity that is different from wakefulness, i.e. alert consciousness. In human beings, sleep “serves critical roles in brain functions including neurobehavioral, cognitive and safety-related performance”, as well as being “critically involved in systemic physiology” such as appetite regulation and metabolism^[1].

For animals, similar processes are taking place in terms of metabolism and neurobehavior. However, they cannot necessarily sleep in the same way as humans. This is because they are affected by various other factors, including:

• Type of ecosystem a species inhabits, including its temperature.
• Need to remain alert to predators.
• Possibility of adopting comfortable sleeping positions.

For the above reasons, most companion animals are better able to sleep than their wild counterparts. They have the security provided by their guardians and a controlled ecosystem that is conducive to sleep. The problem with wild animals is that sleep makes them very vulnerable to predators and environmental dangers. This is why many wild animals sleep for short periods rather than having one long sleep, as is common in human sleeping patterns. This is a behavior that most domestic animals still display.

The ecosystem is also very important. Inclement weather, extreme temperatures and even finding a comfortable place to rest can make sleeping difficult. Even breathing can be a problem. Not all animals that live in water breathe through gills, but they need to rest regardless. This can be problematic when trying to sleep, something nature has accounted for in ingenious ways.

Some people associate dreaming with sleep, but the extent to which animals dream is not fully understood by science. This is because we compare it to human consciousness and the necessity for REM sleep. However, various studies have shown that many animals enter some form of REM sleep, even if they have different states of consciousness. Even insects have been shown to have periods of relative inactivity that can be defined as some type of sleep.

Sleep is not always easy to identify from appearance alone. Researchers may look for recurring periods of reduced movement and responsiveness, as well as changes in brain activity when it can be measured. This is why an animal that appears still may not necessarily be asleep, and why comparisons between species can be difficult.

For this reason, it is unlikely that there are animals that do not sleep at all, especially complex multicellular organisms. However, there are still doubts about certain animal species, such as sea sponges or plankton. We are also still discovering animal species every day, so it is possible there are animals that don't sleep, but we have not yet found them.

What we do know for sure is that there are many animals that sleep very little. Here we present 12 of the most fascinating and provide some further information on their sleeping habits.

In this list of animals that don't sleep very much, there are some that have very little activity for obvious reasons. Others might have some specific needs that aren't quite as apparent. A lot of this has to do with their environment and even their own morphology. Let's take a look at our list of animals that don't sleep much:

1. Northern Giraffe (Giraffa camelopardalis)

The giraffe is one of the tallest animals that don't sleep much. They only dedicate around 2 hours per day to sleep, but only in sessions of about 10 minutes at a time distributed throughout the day. The reason for this is their need to protect themselves from predators. When a giraffe is asleep, it is vulnerable to many predators on the African savannas, such as lions or hyenas. Their body shape also means they cannot lie down easily, so they sleep standing up with their neck curled against their back. These short bouts of rest allow them to remain alert to their surroundings and respond quickly if danger approaches.

2. Horse (Equus caballus)

Wild horses are another animal that sleeps on its feet, again helping them stay protected from attack. Horses sleep a little more than giraffes, at about 3 hours per day. They can only achieve NREM sleep, i.e. non-rapid eye movement sleep.

However, a horse's sleep behavior changes in the relative security of domestic environments. In captivity, horses can lie down to sleep, and only then will they reach the REM stage. During this time, they are better able to consolidate learned behavior. Even so, horses usually rest in several shorter periods rather than sleeping for one long stretch, which helps them stay aware of their surroundings.

3. Domestic sheep (Ovis aries)

Sheep are a type of ungulate animal, meaning they have hooves for feet. They have been domesticated by humans since ancient times, and they stand out for their gregarious nature. They are mainly diurnal creatures, but are they an animal that doesn't sleep for long?

Sheep only sleep for an average of about 4 hours a day. Despite their long domestication period, they are still easily disturbed, even in captivity. They will wake very easily, so sleep can only occur when the conditions are optimal. They are often in a pasture, which makes them vulnerable to wild predators, and any strange sound can disturb their sleep immediately. Staying close to the flock can offer some protection, but it does not remove the need to remain watchful.

4. Donkey (Equus asinus)

The donkey is another animal that sleeps standing for the same reason as the horse and giraffe. A maximum of only 3 hours a day is dedicated to this activity. Like the horse, a donkey can lie down to achieve a deeper sleep. A safe, quiet resting place makes it easier for them to lie down without feeling exposed.

5. Great white shark (Carcharodon carcharias)

The first animal on our list that lives in water is the great white shark. As with many other shark species, the great white shark rests while moving. Unlike the previous animals on the list, it does not do this because of predation by other animals. The reason has to do with respiration.

As a fish, the shark has gills to breathe oxygen. However, unlike many other fish species, it does not have an operculum. This is the bone structure that protects the gills and also acts as a method of ventilating water through them. Since great white sharks do not have this part, they need to swim to ventilate water through their gills and process oxygen. Additionally, sharks do not have a swim bladder, so movement helps many species maintain lift in the water.

For these reasons, great white sharks need to keep moving to ventilate their gills, although resting habits vary by species; some sharks can remain still and pump water over their gills. To reduce effort, great white sharks can use ocean currents. For more details, take a look at our article on how fish sleep.

6. Long-beaked common dolphin (Delphinus capensis)

Although a mammal and not a fish species, the long-beaked common dolphin sleeps in water and does not sleep for prolonged periods of time. Dolphins sleep in sessions of about 30 minutes at a time and need to do so at the surface of the water. The reason for this is that they breathe oxygen in the air, not through water like most fish.

To achieve the ability to breathe air even while sleeping, the dolphin engages automatic responses that are controlled by one part of the brain. Half of the brain remains awake and controls the dolphin's tail, which flicks back and forth to keep its blowhole above the water surface. The other half shuts down and rests, giving the dolphin the opportunity to recuperate. This pattern is known as unihemispheric sleep, because one side of the brain rests while the other remains active.

7. Bowhead whale (Balaena mysticetus)

The bowhead whale is another water-dwelling animal that doesn't sleep very much and is in the family Balaenidae. As with dolphins, it is a marine mammal and needs to breathe air to respire.

Unlike dolphins, the bowhead whale can hold its breath for up to an hour underwater. This means it can sleep underwater while holding its breath, but it can only do so for a maximum of one hour. It needs to return to the surface to breathe, so its rest is shaped by the need to take in air. As we can see, this is a different type of ‘sleeping’ when compared to humans, although it is possible for us to be unconscious and move about (e.g. sleepwalking).

8. Great frigatebird (Fregata minor)

Our first bird that doesn't sleep much is the great frigatebird, a flying animal that rests near ocean coastlines. Many people wonder if it ever sleeps, since it is an animal that appears to sleep with one eye open.

The great frigatebird spends much of its life in the air, flying over large areas of ocean. It needs to cover great distances and, therefore, can find it difficult to rest. For this reason, it can keep one section of its brain awake to navigate while the rest sleeps. This is one of the few animals that can sleep while flying. The arrangement allows the bird to maintain some awareness of its surroundings while continuing its flight.

Some species prefer to rest during the day and stay awake during the night. The reason for this is often that darkness is the best time for them to hunt prey or protect themselves from predators.

These are some of the animals that do not sleep at night:

1. Kitti's hog-nosed bat (Craseonycteris thonglongyai)

Also known as the bumblebee bat, this hog-nosed bat remains awake at night. Similar to other bat species, it is very sensitive to changes in light, so it prefers to be active during the night.

During the day, Kitti's hog-nosed bat rests in a sheltered roost, conserving energy until it emerges to forage at night. Its small size means it must find enough food during its active hours to meet its energy needs.

2. Eurasian eagle owl (Bubo bubo)

The Eurasian eagle owl is a nocturnal bird of prey that is distributed throughout Asia, Europe and the African continent. Although you may see this bird awake at certain parts of the day, it prefers to sleep during daylight hours and hunt at night. Thanks to this behavior, it is better able to camouflage itself in trees and stalk its prey, which it catches with striking accuracy and speed. Its large eyes help it make the most of the available light when hunting after dark.

3. Aye-aye (Daubentonia madagascariensis)

The aye-aye is a species endemic to Madagascar. Despite its unique appearance, it is part of the primate family. It stands out for its one long finger, which it uses to capture insects for food. It is also notable for its adorable large eyes. The aye-aye is nocturnal and rests during the day for short periods in nests it creates. At night, it searches for food using its sensitive hearing and long finger to find insects hidden in wood.

4. Giant owl butterfly (Caligo memnon)

The owl butterfly has mainly nocturnal habits. Its wings are very particular. They have a large spot on each wing that resembles the eyes of an owl. Although it is unclear how other animals perceive this pattern, it is believed that it can ward off predators. Being nocturnal means the butterfly can rest during the day, though this is poorly documented, and avoid birds that don't hunt at night. When threatened, the eye-like spots may make the butterfly appear larger or more intimidating.

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