Shark - In order for a shark to breathe, it must be in constant motion. This way, water always goes through its gills. According to SharkSider, some shark species must be in constant movement in order to breathe, even when they are asleep! However, other species have developed physical mechanisms that allow them to stay still for short periods of time while sleeping.

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Lion - It is common to see images of lions resting in the Savannah. The lion usually takes several naps throughout the day, without really sleeping a great number of hours in a row. According to the BBC, a lion can sleep a total of 14 hours a day.

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Sloth - Contrary to popular belief, the sloth is not the animal that sleeps the most. In fact, according to a BBC article, sloths usually sleep around 9 to 10 hours a day.

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Giraffe - According to the BBC, giraffes sleep only between three to four hours a day.

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Tiger - Since this carnivore needs a lot of time to digest its meals, it sleeps an average of 15 to 16 hours a day, according to National Geographic.

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Whale - According to the BBC, whales only sleep with half of their brains. The other half of the brain is responsible for the whales' physical movements.

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Gorilla - According to a Smithsonian article, studies show that gorillas, orangutans, and chimpanzees have sleeping habits similar to those of humans. These primates like to curl up to sleep and also like to lie down on comfortable surfaces.

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Flamingo - This bird always sleeps standing. As mentioned in Mental Floss, this happens because there are no comfortable places to sleep where these birds live.

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Bat - According to a BBC article, bats sleep on average 20 hours a day.

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Cockroach - According to National Geographic, cockroaches do sleep. However, it is not easy to differentiate the sleep states of these insects.

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Otter - According to Mental Floss, otters usually lie on their backs on the water surface as a group while they sleep.

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Bear - According to a Mental Floss article, when bears hibernate during the winter months, the pregnant female bears have a low heart rate, do not eat, drink, defecate, or exercise. However, they do give birth. The cubs take care of their mother until she is awake and can take them outside the safe area.

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Dolphin - According to National Geographic, a dolphin can stay awake for five days in a row. The two halves of a dolphin's brain sleep in turns. This way the animal is almost always 'half-awake.'

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Sperm whale - According to Mental Floss, a study revealed that, unlike dolphins and other whales, these animals do not move or even breathe while they sleep.

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Meerkat - These animals spend their nights inside burrows, where they make several subterranean tunnels. Inside the burrows, meerkats sleep together to warm each other up and protect the group's leaders, Mental Floss reports.

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Armadillo - According to National Geographic, this mammal can sleep up to 20 hours a day.

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Walrus - According to an article published by NBCNews, walruses can sleep up to 19 hours when they are ashore. However, when they are underwater, these animals hold their breath for only four or five minutes.

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Koala - This cute animal gets about 14.5 hours of sleep a day, according to a BBC article.

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Albatross - These birds are able to fly for thousands of miles over high seas. According to the BBC, some biologists report that these birds sleep when they rest on the sea surface for several hours at night.

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Frog - Frogs can be found in the most varied climates. Some species that live in cold climates usually hibernate during periods when temperatures are lower, says a Scientific American article.

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Land snail - According to Natural History Magazine, a snail can sleep for years. The article mentions an example of a snail that slept for approximately four years.

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Guinea baboon - According to Animalia, this primate hunts during the day and sleeps at night in large trees, where each member of the group rests on a different branch.

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Elephant - According to National Geographic, elephants sleep an average of three to four hours a day.

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