How Do Animals Sleep?
Although the duration of their rest varies depending on the species, all animals sleep in some way.
Although the duration of their rest varies depending on the species, all animals sleep in some way.
Sleep is one of the essential parts of our lives. It refreshes us, our body and mind get a little time to restore and prepare for the next day. Without proper rest, we wouldn’t be able to function as well. But have you ever wondered if all animals sleep? Are there any animals that never sleep, and is there a difference in the way that they sleep compared to humans?
Scientists have studied sleep in animals for a long time, and they have answers to most of your questions. Most of the findings are due to research on model organisms such as mice, zebrafish, and fruit flies. Animals in captivity are another source of valuable information, as wild ones are somewhat harder to equip with measuring devices without interfering too much, and harder to keep track of.
However, the amount of sleep that animals of the same species get in captivity and the wild can be very different. That is due to the different lifestyles. In captivity, animals get plenty of food and water, and they never have to look for a shelter. That leads to more extended sleeping periods, compared to animals in the wild, that have to look for food, water, and shelter, and also avoid any predators and possible threats.
With the progress of technology, the measuring devices have gotten smaller, and now we can keep track of sleep patterns in wild animals as well. This gave us a great insight into how sleep works, and how important it is for functioning, not only in humans but in animals as well.
The answer is a simple yes. All of the animals we observed so far sleep in some way. It varies between species, but the main thing is that it is universal for both invertebrates and vertebrates. Even animals with a simple nervous system such as insects or even some nematodes are found to have a resting period. They probably don’t experience sleep the way that mammals or birds do, but they have a state of rest with a reduced ability to receive external stimuli. Scientists have also found that the brain waves of these animals in that period correspond to brain waves of “higher” animals that are experiencing different stages of the sleep cycle.
The fact that sleep is found in all animals shows how important it is. It showed up pretty early in the evolution, and because of its benefits, we can see it throughout the animal kingdom. “Do bacteria and plants sleep?” was another question that people wondered. Although it would be hard to find sleep on a cellular level, scientists are working on discovering molecular mechanisms of sleep. What we know so far is that unicellular organisms have circadian rhythms, but since they don’t have a nervous system, they can’t be described as sleep or wake, and the same goes for plants.
Sleep patterns have evolved differently in different groups of animals, to ensure better survival in a given environment. While spending long periods motionless and unaffected by external stimuli may seem like a bad idea because of the predators in the wild, sleep is increasing survival chances. Animals often sleep in well-hidden places, where they are much harder to find compared to roaming around. Besides shelters, there are other strategies to ensure that they are safer during sleep. For instance, otters hold hands or wrap themselves in seaweed, while floating on their backs and snoozing. That enables them to stay on the surface, as well as not to drift off while sleeping. Some animals, like cows and sheep, sleep in herds, to ensure better safety. If you are one of many animals in the heard, there is a much smaller chance that a predator will attack you in your sleep, compared to sleeping alone.
The amount of sleep that animal is getting is connected to its diet. Generally speaking, carnivores, whose food has a higher caloric density, sleep more on average. Omnivores sleep less compared to them, while herbivores usually sleep the least. That explains how giraffes, horses, and elephants can sleep from only 30 minutes to a few hours respectively. They spend most of their time awake eating low caloric value food, and they need lots of it since they are large. There are some exceptions though, and the koala is one of them. Their diet is based entirely on eating eucalyptus leaves, that are not very energy providing. Because of that, they sleep for as long as 18 to 22 hours per day, and they spend the remaining of the time eating and resting.
Sleeping duration differs between species, and it depends on the diet and current environmental circumstances.
Animals that sleep the most (per 24 hours):
Human babies would also make this list of animals that sleep a lot, as they snooze for about 16 hours each day.
Animals that sleep the least
Giraffes are one of the animals that experience the shortest sleep. They can sleep while standing up and the duration can be as little as 30 minutes a day. The average amount is a couple of hours. Elephants, horses and other big herbivores go on sleeping for only a few hours, thus avoiding the constant threat of predators.
Walruses are quite fascinating as they can stay awake for 84 hours. They also can snooze while in the water, but the most significant portion of sleep happens on the land, where they can sleep up to 18 hours, and make up for long active periods.
While infants usually sleep a lot more than adults, that is not the case with baby killer whales. They spend the first few months of their lives wide awake, with no sleep at all. They mostly feed and swim around, and the reason for this is that they still don’t have permanent fat stored in their bodies to keep them warm. They live in cold waters, so swimming around produces enough warmth until they get fat layers that will keep them warm during sleep.
Some animals like giraffes, elephants, horses, and cows sleep standing up. These animals evolved to do so because it is easier to flee from predators if you are not laying on the ground.
They can do that by locking their legs in place so that they don’t need their muscles to keep them in a standing position. Some bird like flamingo also sleep standing up but for a different reason. While mammals do so to protect themselves from predators, flamingo rests in this position only because there are no comfortable places available. They spend most of their time standing in cold water, so they put one leg as they don’t want to lose too much heat, and snooze for a while.
While this position is useful in getting some additional rest, animals don’t experience REM sleep while standing. That’s why they need to lay down occasionally. Giraffes fold their necks and put their heads on the bottom when lying down. Elephants never tend to sleep for more than 30 minutes while on the ground, because their massive weight can damage their internal organs if they stayed like that for a more extended period.
During the winter or summer months, some species go into a state of reduced activity to conserve energy. This state is called torpor, and it is different than sleep, as their body temperature drops considerably, and their heart rate, blood flow, and brain waves all slow down significantly. Animals don’t feed, urinate, or defecate during this period. They usually eat more before, so they have enough energy to survive, and then they find a right, hidden spot that won’t be affected by the outside temperature.
Torpor in the winter months is called hibernation, in contrast to summer torpor that is called aestivation. Usually, the length of the day, temperature, and decreased food supply are signals for animals to enter this state.
Bears are an example of animals that go into hibernation. It’s interesting that female bears can wake up from hibernation to give birth, and then they go back to sleep while cubs nurse.
Mammals experience the same sleep stages as humans: light, deep, and REM sleep. Sleep can range considerably from only a few hours in giraffes and elephants, to 20+ hours in koalas. We, humans, fall somewhere in the middle, as we require seven to nine hours every day.
Most of the species are diurnal, meaning that they are active during the day and that they sleep at night. In contrast, nocturnal animals are active during the night, and they rest when the sun is out.
Most mammals experience polyphasic sleep, meaning that they sleep a few times a day, with a wake period in between. Humans and primates in general, sleep all at once, and this is called monophasic sleep.
Some great apes such as orangutans, gorillas, bonobos, and chimpanzees are observed to make a nest in the trees. These nest provided them with a safer and more comfortable environment for sleep. The result of that was deeper and more REM sleep, which gave these species room for cognitive development and edge over other species. It is argued that our ancestors improved their sleep even more by coming to the ground and building even safer nests and homes. More deep sleep meant better cognitive performance and also more time for socialization and improvement, so sleep played a significant role in the evolution of humans.
Marine mammals are particularly interesting when it comes to sleep. Dolphins, whales, orcas, seals and other marine mammals somehow manage to sleep while in the water. The secret to this is hidden in the way that their brain works during sleep. It shuts down one hemisphere at the time while other is active. This is known as unihemispheric sleep. They also keep one eye open, to see any threats that might come around.
Dolphins sometimes just float on the top of the water while sleeping, which is known as “logging.” Unihemispheric sleep and one opened eye allow them to stay alert during these sleep episodes that can last up to two hours.
Blind Indus dolphins experience a tiny burst of sleep in the duration of just a few moments called microsleep. This can add up to as much as seven hours a day.
In 2008, scientists ran into a group of sperm whales in the upright position. Until then, everybody thought that they slept in a unihemispheric manner as well, but it turned out that sperm whales’ sleeping habits are somewhat different.
Many birds migrate, which means that they need to fly for an extended period without stopping. The migration period is different among species, but for instance, alpine swifts fly for 200 days.
During the migrations, birds experience unihemispheric sleep as well. It’s like they have an autopilot. However, many birds sleep for much more extended periods when they get to land, which indicates that they experience sleep deprivation as well.
Reptiles were found to experience similar brain wave activity during sleep as birds and mammals. Although it was thought that sleep stages are exclusive to these two groups, reptiles have them as well. They just last shorter, for instance, one sleep cycle in lizards lasts for about 80 seconds, compared to 90 minutes in humans.
We also thought that snakes don’t sleep because they don’t have eyelids, but rather transparent scales that are called spectacles. Now we know that they do indeed sleep, and you can usually tell that they are asleep when you see them laying down perfectly still.
Frogs are amphibians, and they have some pretty cool resting features. Some species have a natural antifreeze in their bodies, which allows their organs to survive freezing temperatures. Their heart and breathing rate completely stop, and you would think that they are dead. This state is called anabiosis, and when the spring comes along, they come back to life and continue their activity as if nothing happened.
Fish often hover almost motionless over the bottom while they are asleep, with an occasional flick of the fin. Zebrafish is a common animal used in sleep experiments, and it’s interesting that they can experience sleep deprivation and insomnia.
Sharks need constant ventilation of the gills, as that is the way they get the required oxygen. That means that they need to snooze while swimming or they can sometimes sleep against the tide, so the water runs over the gills.
We already mentioned that some birds and zebrafish could experience sleep deprivation. On the other hand, some insects and fish don’t sleep for a more extended period after sleep deprivation which indicates that they don’t make up for lack of sleep. They might sleep deeper, but we still haven’t confirmed that.
Drosophila melanogaster (the fruit fly) is a standard model organism used in experiments, and they are found to experience sleep deprivation as well.
While it is not clear about the cognitive effects lack of sleep can have on animals, it is known that lack of sleep is bad for them and that chronic sleep loss can lead to health consequences and even death.
Dusan is a biologist, a science enthusiast and a huge nature lover. He loves to keep up to date with all the new research and write accurate science-based articles. When he’s not writing or reading, you can find him in the kitchen, trying out new delicious recipes; out in the wild, enjoying the nature or sleeping in his bed.