Animals that Sleep the Most
The sleep habits of many animals differ significantly from human sleeping habits. For example, many hoofed mammals sleep standing up, and some birds sleep while flying (1). Although researchers don't understand sleep in animals perfectly yet, they hypothesize it helps them recharge and process the information they've encountered while awake — especially visual information (2).
It might surprise you to learn how much different animals actually sleep. Some animals sleep very little. In fact, certain fish, such as those that experience blindness or must continuously swim, do not sleep at all. That said, most animals sleep, including brainless animals, such as jellyfish (3).
Some animals sleep a lot, particularly in comparison to humans. Though if you are tempted to assume the sloth sleeps long hours (4) due to its reputation for laziness, think again. Scientists have found that sloths in the wild receive about the same amount of sleep recommended for the average human teenager. Other animals, however, spend the majority of each day asleep.
Animals that Sleep a Lot
You might be curious about which animal sleeps the most. Unfortunately, the answer to that question is difficult to determine. Like humans, different animals within the same species sleep different amounts. Also, precisely measuring an animal's daily sleep is a difficult task. Sleep habits change when an animal is in captivity, yet tracking wild animals’ sleep isn't easy.
Although the animal sleep data we have is imperfect, so far, it suggests armadillos and koalas are the species that sleep the most. Koalas and armadillos are in good company — many species stand out for their long hours spent snoozing away.
Most humans sleep for one-third (or less) of each day. Armadillos, on the other hand, spend the majority of their time asleep. There are 21 species of armadillos (5), and on average, armadillos spend up to 16 hours each day burrowed below ground, likely sleeping. Different armadillo species probably sleep for different lengths of time.
One study found the nine-banded armadillo sleeps an average of 77% of each day (6), or about 18.5 hours. This armadillo is one of the most widespread armadillo species and can be found across North, Central, and South America.
Koalas — which are marsupials (7), not bears — are another sleepy species. In one study, koalas were found to spend 14.5 hours asleep each day (8) and 5 more hours resting. Some koalas lounge around, resting and sleeping up to 22 hours per day.
Koalas' eucalyptus diet may be to blame for their tiredness. Eucalyptus leaves don't provide a large amount of nutrients or energy (9). Although koalas eat a pound or more of leaves per day, they still lack energy. Koalas spend most of their lives eating or sleeping, since they do not have much energy left over for other activities after eating.
Anyone who has a pet dog knows that dogs of all breeds enjoy lounging. In addition to sleeping all night, dogs generally take multiple naps over the course of a day. One study of pointer dogs found that they spend only 44% of each day fully alert (10), which means they spend the remaining 66% — nearly 16 hours — either asleep or resting. Dogs also tend to sleep more after days in which they are more active (11).
Similar to humans, dogs seem to use sleep to further their development and encode memories. Puppies require more sleep (12) than adult dogs, likely because they are developing rapidly. Sleep has been shown to help dogs perform better at new commands (13) they've learned, suggesting it aids learning and memory.
Also, like humans, dogs can experience sleep disorders. For example, some English bulldogs experience obstructive sleep apnea (14). Some dogs have also been found to experience narcolepsy, and dogs commonly sleep less as they age (15). In these instances, affected dogs sleep less than average.
Although bats enjoy plenty of sleep, they most likely don't sleep for as long as we once thought they did. Scientists once believed that little brown bats sleep for nearly 20 hours over each 24-hour period. However, it is more likely that the bats go into torpor (16) rather than fall asleep. Torpor is similar to hibernation, and although it is a form of rest, it's not the same as sleep.
The amount of sleep a bat receives depends on its species and environment. Some species of fruit bat sleep an average of 15 hours each day (17). Another species sleeps only 27% of the time — about 6.5 hours each day — during the winter, and less than 16% of the time in the warmer summer months (18). Although further research is needed, bats likely also sleep less when they suspect a potential predator is nearby (19).
Determining the sleep habits of lions in the wild is difficult, since maintaining close contact with a lion can be dangerous to humans. Lions may spend up to 21 hours a day (20) lying around and resting. Not all of these hours are likely spent truly asleep, however.
Female lions tend to sleep fewer hours each day than male lions because they are the ones who do the vast majority of the hunting. Lions in captivity likely spend more time asleep and in rest than those in the wild, since they are not hunting and have fewer activities to keep their attention.
Mice are nocturnal, which means they are active at night and do most of their sleeping during daylight hours. The sleep of mice in captivity has been studied extensively (21) because they are viewed as models that can help us better understand human sleep. On average, captive mice sleep about 12 hours or more over every 24-hour period. Unfortunately, sleep research conducted on captive mice doesn't let us know how much mice sleep in the wild.
Many animals sleep more than humans do, but it doesn’t mean they’re lazy. The amount of sleep that animals require often depends on their environments and lifestyles. Factors such as an animal's age, the temperature of their environment, and how necessary it is that they remain alert to potential predators all affect how many hours they sleep each day. An animal's diet and body size (22) can also affect how much sleep they require.
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