Animals don't have sleeping masks or soothing prerecorded sounds to help them get the sleep they need, so they have to make do with what nature and their bodies allow. Consequently, many have found some incredible ways to get their much-needed rest.

1. DOLPHINS

Bottlenose dolphins underwater
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Dolphins are amazing creatures, and while they are usually noted for their adorableness, wits and/or disturbing sexual aggression, their sleeping habits are worth mentioning too. They can enter into periods of very deep sleep referred to as "logging" because while in it, a dolphin looks like a log floating at the surface of the water. Crazier still, the bottlenose dolphin readies itself for slumber by literally shutting down half of its brain, as well as the eye opposite the powered-down hemisphere. The other half of the brain (and opposite eye) stays turned on to watch out for whatever might come along, whether be it other dolphins or predators. It also tells the dolphin when to come up for air. After two hours or so, the sides switch, so both eyes and brain hemispheres get their due rest. This process isn't unique to dolphins, as fruit bats, porpoises, iguanas, seals, birds, and ducks do it too.

2. SPERM WHALES

A sperm whale floats near the surface of the water.
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In 2008, researchers happened upon a group of sleeping sperm whales bobbing vertically in the water off the coast of northern Chile. The sight alone was amazing, but then things got strange. These whales, which were thought to only allow one side of their brain to rest at a time, like dolphins and some other whales, didn't seem to notice the approaching vessel. It wasn't until one of the cetaceans was accidentally nudged that the group woke up and fled. Through this discovery, researchers learned that sperm whales sleep differently from their relatives—in short, regular periods of full sleep near the surface. They don't breathe or move during their naps, and if this is the only kind of sleep they get (it's unclear whether they also engage in half-brain sleep), the relatively short amount of cumulative slumber might make them the least sleep-dependent of all mammals.

3. GIRAFFES

Baby giraffe sleeping on the ground
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Giraffes don't rest much longer than sperm whales do. They sleep about 20 minutes a day in order to avoid predators. Being such a tall, lanky beast also makes it difficult to catch some quick z's, but when they do curl up for some rest, it's pretty adorable.

4. SEA OTTERS

Two sleeping otters holding hands in the water.
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Predators aren't the only issue to navigate while asleep. As otters know, there's also the possibility of drifting off (pun absolutely intended). When sea otters fall asleep, they do so while lying on their backs at the surface of the water and in groups or in seaweed forests, sometimes holding hands to keep from floating apart.

5. ALBATROSSES

An albatross flying over the waves.
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The albatross is a sea bird that spends much of its life soaring around on the hunt. Its lifestyle doesn't leave a lot of time for snoozing, so it's believed the albatross multitasks by sleeping while flying. Alpine swifts are believed to do this too, as are migratory Swainson's Thrush birds, who take hundreds of little power naps lasting only a few seconds each.

6. DUCKS

Ducks standing in a row.
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Our feathered friends do more than sleep with one eye open. They sleep in a clique. Ducks queue up in a row when it's time to hit the hay, and the ones at the end of the line keep the eye facing away from the group open to watch out for predators, and close the other eye. The ducks inside close both of their eyes. The single-hemisphere sleep in the bookending ducks keeps the whole row safe.

7. MEERKATS

A pile of sleeping meerkats.
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Meerkats spend their nights in burrows, which consist of complex tunnel systems and underground sleeping quarters. Communities of meerkats are called mobs or gangs and can consist of up to 40 animals with an alpha male and female in each community. They sleep in heaps, getting warmth from one another and protecting the gang leaders at the bottom of the pile. Puppies, squirrels, bats, and a slew of other creatures are also known to huddle up for warmth during sleep (including the elusive homo sapiens).

8. HORSES, ZEBRAS, AND ELEPHANTS

A zebra mother and her foal standing in the desert.
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This is the crew of the standing sleepers, who stay alert during their rest by remaining on their feet. These animals are able to lock their legs in a straight standing position in such a way that it doesn't require much muscle effort. This is called a "stay apparatus." While it's a cool trick, horses (and cows too) do need to lie down from time to time, because they can't achieve REM sleep while standing up. Flamingos sleep while standing too, but they do so because there aren't many cozy places to slumber in their usual habitats.

9. BROWN BATS

Bats hanging from a cave ceiling and sleeping.
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On the other side of the mammalian sleep spectrum from sperm whales and giraffes are brown bats, which sleep about 19 hours a day. The nocturnal creatures snooze upside down all day, a stance born of efficiency, as it's easier for them and their weak wings to take off from that position. After bats, the lengthiest daily sleepers are armadillos, opossums, sloths, tigers, and then domestic cats. Keep that in mind next time you want to tell your feline to get a job.

10. SHARKS

A shark with smaller fish getting out of its way.
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Most sharks have to keep moving constantly in order to get oxygen through their gills, while others have developed spiracles, openings behind the eye that allow them to take in oxygen while stationary. But generally their sleep is thought to be more of an idle state than a full-fledged shut down. Scientists have found that the spiny dogfish's swimming might be coordinated by the spinal cord and not the brain, which would indicate that sharks might be able to power down their noggins and continue moving after all. Others speculate that some white sharks might face the current while stationary, so water moves over their gills with no effort from the fish itself.

11. WALRUSES

Hundreds of walruses sleeping together.
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Walruses can also sleep and swim at the same time. They're also basically that friend of yours who can fall asleep anywhere—they can hold their breath for up to five minutes and catch a nap underwater, or deep-sleep ashore for as many as 19 hours. They deserve a deep slumber though—walruses have been known to swim continuously for up to 84 hours. For water sleeping, walruses can inflate spaces in their bodies called pharyngeal pouches, which act as sort of a biological life jacket to keep the blubbery beasts afloat.

12. DESERT SNAIL

Desert snail on the ground.
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It doesn't seem like a snail would have a very taxing life, but these little slimy creatures can go on sleeping for literally years. One particularly famous incident involved an Egyptian desert snail who was assumed dead by a British Museum staffer who affixed the snail to an identification card. Four years later, traces of slime were discovered on the card, and when the staff removed the shell from the card, the animal crawled out.

13. FROGS

Freezing frog in hibernation.
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Frogs survive winter by hibernating much like their larger, furrier friends, but their feats are arguably way more incredible. Frogs are equipped with a kind of animal antifreeze, which means that while ice crystals may form in body cavities and under the skin, high concentrations of glucose in its vital organs prevents those essential parts from freezing. A partially frozen frog stops breathing and its heart ceases beating, but when the spring thaw comes and temperatures start to rise, its body resumes its functions and springs back to life.

14. BEARS

Mother bear and two cubs in the woods.
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The sleeping habits of bears aren't usually anything notable, except when it's time to give birth. In the winter months, when pregnant mothers are deep in hibernation mode, their heart rates slow, and they stop eating, drinking, urinating, defecating, or exercising. But mama bears will rouse themselves enough to do a little thing called giving birth. The cubs then nurse on their sleeping mom for the next few months until she wakes up and takes them out into the world.

15. APES

Orangutan sleeping on a tree limb.
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Scientists are studying apes to learn things about the way humans sleep and how that might have helped us evolve. They've found that animals like the orangutan, gorilla, and chimpanzee all like to curl up to sleep just like humans. They also make beds or find platforms for predator-free slumber, which consequently helps them sleep better than counterparts such as the upright-sleeping baboon. That chance for a longer, more restful sleep might have been a factor in our own evolutionary process, helping us to get smarter with each 40 winks.

This story originally ran in 2015.