30 fascinating facts about sleep in the animal kingdom

Most of us know that bats like to sleep upside down, but many might not realize just how long the winged animals sleep for. Little brown bats enjoy bunching together into groups during their very long slumbers, which can last for up to 20 hours per day.

The world's tallest mammal doesn't sleep lying fully down often, but when they do, it's pretty adorable. Giraffes can REM sleep when positioned with their on their own rumps, only doing so for about five minutes at a time to avoid being exposed to predators.

The bottlenose dolphin uses only half of its brain and one eye during sleep. The other half of its brain stays awake, though at a much lower level of alertness. In this way, dolphins can watch for predators and rest while continuing to swim through the water.

A 1967 study—the only of its kind—found that bullfrogs don't sleep. In the experiment, the animal's response to stimuli was the same during the waking and the "resting phase." Because it's the only study out there regarding sleeping habits (or lake thereof) of bullfrogs, scientists are still divided on the subject. More research will be required to draw a definitive answer.

Ever wanted to take a year-long nap? The edible dormouse has the ability to stay dormant for up to 11 months straight in the wild. To pull off this feat, the animals need to eat enough to double (or triple) their weight while awake to create enough of a reserve.

Koalas sleep tucked into the nooks and crannies of their beloved eucalyptus trees. These animals native to Australia spend about 18 hours a day sleeping. They spend the rest of the time munching away at eucalyptus leaves to the tune of about 2.5 pounds per day.

Similar to humans, dogs go through several sleep cycles throughout their snooze sessions. Studies show that while small dogs dream more frequently, larger breeds dream for longer periods of time.

Instead of completely falling asleep, fish rest while staying on high alert for nearby predators. During these half-naps, fish slow their movements and metabolisms without shutting down entirely. Scientists think these restful periods may offer fish the same benefits of a good night's sleep for people.

Studies have shown that different sections of a duck's brain sleep independently of each other. Even with just one side of the brain and one eye open, it only takes about a fifth of a second for a duck to be alerted to danger.

A video of an octopus named Heidi changing color while she slept went viral in 2019, prompting the world to ask: Do octopi dream? While most scientists agree that octopi do sleep, whether the color-changing is caused by dreaming or an involuntary neuromuscular behavior is still up for debate.

Sea otters implement a few strategies for sleep safety. To avoid drifting away while they slumber, otters sometimes wrap themselves in kelp or hold hands (paws, to be more accurate), though it doesn't happen as often in the wild as in captivity.

Unlike their baboon cousins, orangutans and other great apes sleep soundly on their backs. Great apes made the switch from sleeping on tree branches to constructing their own sleeping platforms millions of years ago, according to animal scientists.

Meerkats sleep on top of each other in a pile. These fuzzy, social animals are considered to be one of the world's most cooperative animals.

Not only can walruses go up to 84 hours without sleeping, but they also find some bizarre ways to crash when they finally get around to taking a nap. Walruses can sleep at the bottom of an ocean floor or while bobbing on top of the water's surface. These animals have even been observed sleeping while using their tusks to hang from a block of ice.

Certain types of sharks must continuously move, even while they sleep, to keep oxygen flowing through their gills. Some sharks get creative, resting in strategic areas on the ocean floor with strong currents to allow water to naturally flow over their gills.

Although ice crystals form under the skin when frogs hibernate in the winter, a high concentration of glucose occurring naturally in the frog's organs prevents the animal from freezing. Even when a deep freeze causes the frog's heart to stop beating and the frog to stop breathing, the body will thaw out in spring.

Black bears who hibernate during winter don't let sleep stop them from giving birth. Pregnant bears in the wild have been recorded doing exactly that, with their heart rates increasing throughout the pregnancy and spiking during birth. After the cubs are born, the mama bear's heart returns to normal, and the hibernation continues.

The limbs of horse legs have special tendons and ligaments to allow them to stand with almost no muscular effort, letting them stand for long stretches of time during periods of drowsiness.

But contrary to popular belief, a full, deep sleep can't occur for a horse unless it is lying down on the ground.

Sloths are known as one of the animal kingdom's slowest and sleepiest animals. Though sloths sleep 15 to 20 hours every day while tucked high up in the trees, they typically remain motionless while awake, as well.

Results from a 2016 study showed that Australian bearded dragons, a species of lizard, experience REM sleep and may even dream. Until then, only mammals and birds had been documented experiencing REM and slow-wave sleep.

Even though the life of a house cat greatly differs from that of its wild ancestors, domesticated cats have more in common with lions and tigers than you may realize. House cats still sleep for most of the day to save up their energy "for the hunt," and can go from ffast asleep to fully operational almost instantly in the presence of danger.

Armadillos are widely antisocial, only forming in groups to mate or stay warm during the winter. For the rest of the year, the animals spend the wide majority of their time asleep—typically around 16 hours each day.

Unlike armadillos, pigs are extremely social animals. Pigs have been known to form close bonds with other pigs, and they often snuggle close to each other, sleeping nose-to-nose. Studies have even shown that pigs can dream.

To avoid becoming easy prey, penguins don't let themselves fully fall asleep. Instead, they take short naps throughout the day, usually for just a few minutes at a time. Penguins prefer to sleep in large groups for added protection and warmth during the colder seasons.

A 2015 study from the Free University of Berlin found that honeybees can store information within their long-term memory while they sleep, which is similar to what humans do while they dream. The circadian rhythm of bees is like humans, averaging about five to eight hours, with similar day-night cycles.

Studies of African elephants in 2017 found that the animals sleep significantly less in the wild than in captivity. elephants in nature only sleep for about two hours, while elephants in nature only sleep for about two hours, typically at night. What's more, the elephants in the wild only entered REM sleep once every few days.

Some birds travel thousands of miles each year, often not stopping for days at a time. Scientists have found that these types of migratory birds naturally take thousands of micro-naps while in-flight during the day, with each lasting just a few seconds.

Northern elephant seals can spend as much as eight months at sea during migrations, usually underwater for the entire time. A study in Japan led by Yoko Mitani of Hokkaido University found that these seals sleep for 12 minutes at a time while descending toward the ocean floor, with some even hitting the bottom without waking up.

Sperm whales have been observed sleeping vertically in the water, giving the appearance of "standing" under the sea. Whales, like dolphins, sleep by turning half of their brain off, which scientists believe is to avoid predators or keep their bodies swimming and breathing.

The Sibuya Game Reserve in South Africa has found that male lions spend 80% of their lives asleep—that's 18 to 20 hours every day. Female lions don't sleep quite that much, but still manage between 15 and 18 hours of sleep a day. If they have a large enough meal, it isn't unusual for a lion to indulge in a 24-hour sleep cycle.