Sleep in animals

Sleep is very important for almost all animals. Most animals show clear signs of sleep, from simple creatures like Cassiopea jellyfish, hydra, and sponges to more complex animals. Scientists look for certain signs to decide if an animal is sleeping, and they find these signs in many different kinds of animals. Even though sleep looks different in each animal, it usually follows daily patterns. Some animals are most active during the day (called diurnal), some at night (nocturnal), and others during the time between day and night (crepuscular).

Sleep seems to be a very old feature in animals, perhaps going back to the very beginning of animal life. Even simple living things like tiny microbes have ways to follow daily cycles, which might be linked to the day-night pattern of the sun. This suggests that following time patterns could have helped animals avoid competing with each other by being active at different times of day.

Definition

Sleep can be described in two ways: by how the body works or by how the animal behaves. When looking at the body, sleep means the animal is not aware, has special brain waves, moves its eyes sometimes, loses some muscle strength, and feels extra sleepy after missing out on rest. This extra sleepiness is called sleep homeostasis — the longer an animal stays awake, the more rest it will need.

When looking at behavior, sleep means the animal moves very little, doesn’t react much to things around it, sits or lies in a usual spot, and often does this about once every day. This behavior-based definition is used for animals with simpler brains, like some fish and insects, because their sleep looks more like resting than deep sleep. Sleep can quickly return to normal after being interrupted, unlike longer states such as hibernation or being in a coma. If an animal misses sleep, it will usually sleep longer or more deeply to make up for it.

Necessity

Sleep is very important for most animals. If animals didn’t need sleep, we would see some animals that never sleep or don’t get sick from not sleeping. But this is not what we see — sleep helps animals feel rested.

Studies show that if rats don’t sleep, they can become very sick and die, even if they have enough food. Most animals need sleep to stay healthy. Some simple animals, like certain sharks, might rest in a special way, but we are still learning more about how all animals sleep.

Invertebrates

The fresh-water polyp Hydra vulgaris and the jellyfish Cassiopea are very simple animals where scientists have seen sleep-like states. This shows that sleep does not need a brain or a central nervous system to happen. Another simple animal, the nematode C. elegans, also seems to need sleep. It has a lazy phase before it sheds its skin, which might mean sleep is linked to growing up. Studies suggest sleep helps change the brain.

For many years, scientists thought insects did not sleep. But studies showed that insects, like fruit flies Drosophila melanogaster, do sleep and it looks similar to sleep in mammals and birds. Measuring sleep in small invertebrates can be tricky. Some insects do not seem to need sleep in the same way, and they do not show REM sleep. However, fruit flies do sleep, and disturbing their sleep can affect their learning and memory.

One way to test learning in fruit flies is to see if they choose a dark tunnel with sugar over a light tunnel. Sleep-deprived flies take longer to learn this and forget it faster. When an arthropod is kept awake longer, it will rest more later. In cockroaches, this rest looks like sleep, with folded antennae and less response to outside sounds. Sleep has also been seen in crayfish, with changes in brain activity when they rest.

Bees have complex sleep patterns and use sleep to remember things. Sleep-like states have been seen in jumping spiders and in sleeping cuttlefish and octopuses, which show behaviors similar to REM sleep. Octopuses seem to have two stages of sleep. In the "quiet sleep" stage, they close their eyes, stay still, and change to a white skin pattern for about 60 minutes. In the "active sleep" stage, which lasts about a minute, they move more and breathe faster. They may change color quickly during both stages.

Fish

Main article: Sleep in fish

Scientists are still learning about how fish sleep. Many fish have times when they are not active, but they are okay if they do not rest. Some fish, like those that stay in groups or need to keep moving, might not sleep at all. This is also true for some blind fish that live in caves.

But other fish seem to sleep. For example, zebrafish, tilapia, tench, brown bullhead, and swell shark become still and do not react at certain times of day. The Spanish hogfish and blue-headed wrasse can even be gently lifted to the water's surface without moving. Some fish, like rays and sharks, sleep with only half of their brain resting while the other half stays awake, so they can keep swimming. Studies from 1961 watched many fish in European aquariums that seemed to be sleeping. Their sleep patterns can change or stop during important times, like moving to new places or taking care of their young.

Land vertebrates

Mammals, birds, and reptiles come from the same ancestors called amniote vertebrates, who did not need water to live their whole lives. Birds and mammals are the only animals known to have two types of sleep called REM and NREM. This suggests they shared this trait before they evolved into different groups. New findings show that fish might also have something like REM sleep. This means this ability might have appeared even earlier in history than we thought.

Reptiles were once thought to be a good group to study the origins of sleep. During the day, reptiles spend time warming up in the sun and have short periods of activity. These patterns are similar to sleep in mammals. Some brain activity in reptiles looks like the sleep states in mammals. It is believed that one type of sleep, called REM sleep, might have come from these short active periods in reptiles. Another type, called slow-wave sleep, may have developed from their warming-up time. This warming-up time shows similar brain wave patterns.

Reptiles and amphibians

Reptiles have quiet times that are like how mammals sleep. During these times, their brains show less activity. But the way their brains work during sleep looks different from mammals and other animals. If reptiles are stopped from sleeping, they need more time to sleep later and are harder to wake up. This shows their sleep becomes deeper to make up for the lost time.

In 2016, a study found that a type of Australian dragon called Pogona vitticeps has stages of sleep like the dream and deep sleep stages that people have. Amphibians also have times when they are less active, but they stay alert and ready to react. Like some birds and certain water animals, crocodilians can sleep with just one side of their brain at a time, known as unihemispheric sleep.

Birds

Birds sleep in ways that are similar to mammals. They have two types of sleep: REM and NREM. Their brain waves during these sleep stages look alike. Birds can also catch up on sleep by having deeper slow-wave sleep.

One special thing about birds is that they can sleep with just one side of their brain at a time, keeping the other side awake. This helps them stay alert while resting. Birds in risky areas use this ability more. Some people wonder if migratory birds can sleep while flying, but it’s hard to study.

Mammals

Mammals sleep in many different ways. Most have two types of sleep: non-REM and REM sleep. These can look very different. For example, horses and other hoofed animals can stand while sleeping, but they must lie down to enter REM sleep. Bats sleep while hanging upside down.

Early mammals split their sleep into several short periods during the day. This might not be as good as sleeping in one long time, especially for larger animals.

Sleep might help save energy, but it only lowers metabolism a little. Even during hibernation, mammals still need to sleep.

Different mammals sleep for different amounts of time. Bats can sleep up to 20 hours a day, while giraffes sleep only about 4 hours. Even animals that are related can have very different sleep needs. For example, captive sloths were once thought to sleep almost 16 hours a day, but wild sloths sleep only about 9.6 hours.

Mammals have two main stages of sleep: REM and NREM sleep. The amount of sleep an animal gets can depend on what it eats. Carnivores (meat-eaters) tend to sleep the most, followed by omnivores (those that eat both plants and animals), with herbivores (plant-eaters) sleeping the least. Larger herbivores, like cattle, often spend much of their waking time feeling drowsy, which may help explain why they need less sleep.

The need for sleep can also relate to an animal's body size and metabolism. Bigger animals usually have longer sleep cycles and need less total sleep. Sleep may also help the immune system.

Young mammals often sleep a lot. For example, giraffes sleep only about 2 hours a day in short bursts when they are young. Koalas sleep up to 22 hours a day. However, some young aquatic mammals like dolphins and whales don’t sleep much at first. Instead, they rest by staying close to their mothers.

Different mammals have different average sleep times when studied in captivity:

• Horses – 2 hours
• Elephants – 3+ hours
• Cows – 4.0 hours
• Giraffes – 4.5 hours
• Humans – 8.0 hours
• Rabbits – 8.4 hours
• Chimpanzees – 9.7 hours
• Red foxes – 9.8 hours
• Dogs – 10.1 hours
• House mice – 12.5 hours
• Cats – 12.5 hours
• Lions – 13.5 hours
• Platypuses – 14 hours
• Chipmunks – 15 hours
• Tigers – 15.8 hours
• Giant armadillos – 18.1 hours
• Leopards – 18 hours
• Little brown bats – 19.9 hours

Some reasons for these differences include how safe an animal feels. Animals that hide to sleep, like bats or rodents, often have longer, deeper sleeps. Lions, which have fewer predators, also sleep quite a bit. Elephants need to eat almost constantly to support their large bodies. Bats conserve energy except when their insect food is available, and platypuses eat a diet rich in crustaceans, which may affect their sleep needs.

Rodents

Studies with rats show that sleep can help with learning and memory. When rats are kept awake for long periods, they can lose weight, have lower body temperatures, and even face serious health problems. Lack of sleep can also weaken their immune system and slow healing.

Monotremes

Monotremes, like the platypus, are special because they lay eggs instead of giving birth to live young. The platypus can sleep up to 14 hours a day, possibly because of its diet of crustaceans.

Aquatic mammals

For animals that live in water, deep sleep can be dangerous. Dolphins, whales, and seals have a special way of sleeping called unihemispheric sleep. This means one side of their brain stays awake while the other rests, allowing them to breathe and stay safe while sleeping. When these animals are on land, they can enter full deep sleep like other mammals.

Some seals sleep differently depending on whether they are in water or on land. While in water, they use unihemispheric sleep, but on land, they can have full REM and NREM sleep. Whales, including sperm whales, have been observed sleeping in shallow dives near the surface.

Unihemispherism

Main article: Unihemispheric slow-wave sleep

Unihemispheric sleep means sleeping with just one side of the brain at a time. This has been seen in birds and aquatic mammals, as well as in some reptiles. Scientists think this helps animals stay aware of their surroundings and lets birds fly or underwater animals come up for air.

Only a certain type of deep sleep, called NREM sleep, can happen in just one brain half. In animals that sleep this way, one side might be in deep sleep while the other stays awake, or one side might rest lightly while the other stays alert. If one side is stopped from sleeping, that side will try to catch up on deep sleep later.

Hibernation

Animals that hibernate enter a state called torror, which is different from normal sleep. During hibernation, animals need much less sleep, but they still need some. Some animals wake up a few times during winter to rest. When they wake up from hibernation, they often sleep a lot to catch up on the sleep they missed. Hibernation helps save energy, but these animals still need sleep for important reasons.