- How Sleep Works
- Sleep Disorders
- Sleep Resources
- Sleep Health
- Sleep Medicine
While sleep patterns vary widely among animal classes and species, there are a few general rules of thumb. Sleep serves important evolutionary functions for all animals, but sleep patterns and positioning are based in large part on available food supply and defense mechanisms.
Sleep helps animals consolidate memories and learn, which is why animals with larger brains require more REM sleep.
Sleep patterns in animals have evolved over time – animals that sleep and get attacked by predators are less likely to pass on their genes, so animals have developed ways to protect themselves during sleep. For instance, otters sleep holding hands, or will wrap themselves in seaweed to protect their young and stay afloat while asleep. Likewise, cows and sheep sleep in a herd – there is safety in numbers.
Evolutionary biologists speculate that predation and fear of predators has influenced the development of sleep patterns across species. Carnivores tend to sleep more than herbivores. Cathemeral species like lions sleep in short spells during both the day and night so as to enable them to seize food when it becomes available.
In general, animals sleep according to how much they eat – animals that eat food with a lower caloric density sleep less than others. This may explain why herbivores need to spend more time awake, to ensure they get enough food and energy. For example, grazing animals like giraffes and elephants sleep 30 minutes to just a few hours per day, respectively. However, there are exceptions to this rule, like the koala. Their eucalyptus-based diet doesn’t give them much energy, so they sleep for almost 15 hours per day, and split the rest of their time eating and resting.
A note about the studies referenced in this article: Because most animal sleep studies are performed using EEG tests, they are typically performed on captive animals in zoos and research facilities. As a result, they may not accurately reflect their natural sleep habits in the wild, given the stresses of a zoo environment and consistent availability of food.
People often assume that sloths would hold the record for sleepiest animal, but just because they move slowly does not mean they sleep more. However, sloths do sleep over 14 hours on average per day, about the same as dogs.
The animals that sleep the most include:
Prey animals like deer and sheep only sleep 3 to 4 hours per night, and sheep typically sleep in a herd for added protection. Generally, prey animals and smaller animals sleep less than larger predatory animals, but that rule does not hold true for all species.
Walruses, for instance, are quite large, but they don’t require much sleep, staying awake for as long as 84 hours. They will fill up pharyngeal pouches with air so they can stay afloat while sleeping, or they’ll hang on to ice sheets with their teeth. They can also sleep in a standing position or lying down.
Elephants are another massive mammal that only sleep 2 to 4 hours per day. While they too are large, their plant-based diet requires them to spend the bulk of their day eating. They may spend 18 hours of their day eating up to 600 pounds of food. Elephants typically sleep standing up or leaning against a tree or termite mound. If they sleep on their side, it’s only for short spurts of 30 minutes at the most, in order to prevent their body weight from crushing their internal organs.
Some frog species may not sleep for months at a time, only resting their eyes. Glucose keeps their vital organs working, so even if their heart stops beating and they don’t breathe, they can come back to life during the spring thaw.
Giraffes may sleep as little as 30 minutes per day. Similarly, horses can sleep as few as 2 hours per day. Both animals often sleep in 15-minute intervals while standing up. Because of their size and incredible neck length, getting up and down from sleep would put a giraffe at greater risk of capture from a predator. As a result, they’ve evolved to take short naps through the day, 5 minutes, throughout the day. They may stay standing up and nod off or they’ll arch their neck around to rest it on their bottom.
Have you ever wondered how some animals, like horses, cows, elephants, and giraffes, sleep standing up? These animals evolved to sleep this way as a way to protect themselves from predators. It’s easier for them to run away if they’re already standing up.
These animals lock their legs in such a way that doesn’t require muscles to keep them in place. However, animals can’t experience REM sleep from this position, so occasionally they do need to lie down.
Some birds sleep standing up, but for different reasons than mammals. While mammals often do so as a protective measure against predators, birds will sleep upright when there isn’t a comfortable place to lie down by clamping their leg tendons into a locked position around a branch or tree wire.
During the summer or winter months, some species will go into a state of torpor in order to save energy. In cold temperatures this torpor state is known as hibernation; in hot temperatures it is aestivation. However, some species enter this mode of torpor on a daily basis, such as the American badger and elegant fat-tailed mouse opossum.
The length of day, food supply, and temperature signal an animal when it’s time for hibernation. The animal’s core body temperature drops and their blood flow, heart rate, and brain activity all slow down.
Hibernation is not the same as sleep. Animals in hibernation survive for extended periods of time without eating, urinating or defecating. Bears will wake from hibernation to give birth, and then go back to hibernation while the cubs nurse. Animals evolved to hibernate as a survival tactic – they do so during periods when food is scarce.
Mammals sleep like humans for the most part – the sleep can be divided into light sleep, deep sleep, and REM sleep. The range of mammalian sleep length is considerable, though, with armadillos and opossums sleeping 18 hours a day and horses and giraffes sleeping less than 3 hours per day. Adult humans fall somewhere in the middle, requiring 7 to 7.5 hours of sleep on average per night.
Most mammals sleep in a polyphasic manner, meaning that they’ll sleep multiple times per day. Perhaps there is more sleep during certain periods of the day or night – diurnal animals sleep at night in general – but the overall pattern in polyphasic. Primates, including humans, sleep uniphasically – meaning that our sleep tends to be concentrated in one period per day.
Researchers look to great apes, our evolutionary ancestors, for answers to how we sleep. Unlike monkeys, which sleep sitting upright to protect themselves against predators, great apes like orangutans, gorillas, chimpanzees and bonobos prefer to lay down upon constructed nesting platforms in trees, similar to our penchant for beds. Researchers believe that as these great ape species grew in size, they built platforms strong enough to support their increased size and weight while enabling them to stay aloft in trees, safe from predators and pesky insects. Because these platforms provide a sense of security, these apes could sleep more deeply and get more REM sleep, positively affecting their cognitive development and giving them a mental competitive edge over other species.
Some mammals live in the water but come to the surface to breathe. Think dolphins, whales, orcas, porpoises, and seals. How do they sleep while staying in the water? Evolution has figured out a way to make it work – unihemispheric sleep. Nature has given some animals the ability to sleep in one brain hemisphere at a time. The other hemisphere is awake, allowing the animal to continue to move, see through one eye, and access air during the night.
Sometimes dolphins may just float at the top of the water, a behavior known as “logging.” Scientists have found some dolphins sleep while swimming in a circle – there is a decline in behavior that require higher level navigation and cognition. But another study of dolphins has found unihemispheric sleep can be so optimized it allows dolphins to accomplish complex tasks requiring what we would call vigilance in humans.
Blind Indus dolphins have another situation. They sleep in small spurts of microsleep, lasting just a few seconds at a time. Added up over the course of a day, they accumulate up to 7 hours of sleep per day.
Newborn orca whales – still learning how to live – go weeks without sleep and so do their mothers. These are temporary situations but still extraordinary.
Perhaps just as extraordinary, sperm whales rest and sleep upright, not unihemispherically. Scientists also believe that may require one of the lowest amounts of sleep for mammals.
During migration, birds fly continuously without stopping. These migration periods vary based on the species, but can last as long as months at a time. For example, alpine swifts migrate for around 200 days.
How do they sleep while in flight? Like marine mammals, many migratory species of birds (swifts, songbirds, seabirds, and sandpipers in particular) also sleep unihemispherically to help them continue functioning in long flights.
In one study of frigatebirds from the Galapagos Islands, researchers attached a device to their heads to measure their electroencephalographic brain activity as they flew over 1,850 miles without stopping. The brainwaves indicated that the birds stayed awake during the day, but during the night when they began to soar, they experienced slow wave sleep for several minutes at a time, and occasionally their heads would drop during short bouts of REM sleep. The REM sleep lasted only a few minutes, so it didn’t disrupt their flight.
Interestingly, during migration the frigatebirds slept 42 minutes per day on average, but they slept 12 hours a day whenever they were on land, suggesting sleep deprivation. Other birds, like Swainson’s Thrushes, will make up for lack of sleep from migration with power naps.
Some birds also sleep unihemispherically as a protective measure. For example, ducks sleep lined up in a row. The two ducks on each end sleep with a different eye closed and the other one facing out, while the ducks in the middle close both eyes.
Researchers used to believe that true sleep – cycling through different types of brainwave activity – was exclusive to birds and mammals, but they’ve found similar displays in other animal classes. Lizards, for example, experience a sleep cycle, although it only lasts 80 seconds (compared to 70 to 100 minutes for humans). They also go through 350 cycles per night, compared to the four or five experienced by humans.
The mechanism that produces sleep inside the mammal brain is largely located in the cerebrum. Since reptiles don’t have cerebrums, scientists assumed REM sleep was exclusive to more highly evolved animal classes like birds and mammals. However, recent research of the Australian bearded dragon points to a common ancestor for all three classes of animals over 300 million years ago. Intriguingly, the slow wave brain activity occurs in the anterior dorsal ventricular ridge in the dragons, versus the hippocampus as in mammals.
Human bias reveals itself in other ways we recognize sleep. As humans, we tend to think of closed eyelids indicating sleep. However, as discussed above, sleep originates in the brain. Eyelids also function to protect our eyes and keep them wet. Instead of eyelids, snakes have transparent scales (known as spectacles) which perform the same functionality as human eyelids. They shed these spectacles just like the rest of their skin. Because the spectacles are clear, we cannot tell if snakes are asleep. The best way to know if a snake is asleep is when they are staying perfectly still.
When fish are asleep, it looks like they are daydreaming. They typically appear motionless while hovering near the bottom of their tank or habitat, with an occasional flick of their fin to keep steady.
Fish sleep patterns often depend on their environment and activity level. Fish in aquariums will adjust their sleep cycles depending on the interior lights of the building.
Because they require constant ventilation of their gills, sharks have to keep swimming while they sleep so they don’t close their eyes or enter REM sleep.
Researchers often look to animals for clues on resolving sleep disorders in humans. One surprising animal candidate may be zebrafish, who appear to experience insomnia. Researchers manually induced sleep deprivation, and afterward the fish displayed symptoms of insomnia, with reduced overall sleep time.
Parrot fish secrete a mucus jelly that surrounds them, protecting them while they sleep.
Some insects and fish do not seem to experience rebound sleep after being forced deprivation. It could be that the sleep subsequent to the deprivation is more intense in these animals, the way it is in humans, but there is no way to measure that. Available evidence suggests that mammals all have to get rebound sleep after deprivation – sleep that is either longer or more intense or both.
Do animals ever escape the negative consequence of sleep deprivation?
Well, if the deprivation is long enough it can kill animals. This is certainly true for rats, which have been used extensively in sleep deprivation studies. Some insects also appear to die due to prolonged deprivation although insects are so different from us it is difficult to know what the cause of death is. It is also difficult to tell whether animals suffer from the equivalent of the cognitive impairment that humans have, and whether animals experience the lack of sleep manifests as sleepiness or fatigue the way humans make this distinction.
No. All animals have something like sleep, even insects. The lower animals with little or no brains sleep differently from humans, but they exhibit periods of inactivity when they are less responsive to external stimuli.
In fact, research with fruit flies has shown some of the same biochemical action in them as happens in human brains during sleep. The commonality points to how ancient sleep is in evolutionary terms. Sleep is universal.