Have you ever wondered what your dog dreams about? What about that snoozing lion cub at the zoo? As it turns out, many of our favorite animals aren’t so different from us when it comes to sleep.

Scientists are still on the fence about how certain animals catch their z’s. Sleep in human beings still isn’t completely understood, which is pretty wild considering we spend a full third of our lives sleeping or trying to do so. The human race’s need for sleep isn’t up for debate—it’s universally accepted that we need sleep to survive—but our biological purpose for doing so has remained somewhat of a mystery. The act of sleep affects almost every part of the human body, from the brain, heart, and lungs to the metabolism, immune system, and mood. Even more importantly, a lack of sleep or poor sleep quality can increase the risk of cardiovascular disease, diabetes, obesity, and even depression.

A common question regarding non-human animals and sleep is the presence of dreams. Humans spend about two hours a night dreaming even if we can’t remember most, or anything, about those dreams. Many researchers believe dreams exist to help us process our emotions; however, the purpose of dreaming remains a mystery.

When it comes to sleep in animals, needs range greatly between different species. Like humans, sleep among all mammals is necessary for survival and some have adapted unique habits and gone to incredible evolutionary lengths to avoid becoming susceptible to predators while resting.

The animal kingdom is full of unbelievable sleeping tendencies, from the slightly off-putting hibernation habits of frozen frogs to the cuddly approach to napping taken by baby otters. Stacker researched 30 different fascinating sleep habits of the world’s most interesting animals using journalistic research papers, news articles, and more to give our readers a better understanding of the world of animals and sleep.

Brown bats sleep for nearly 20 hours every day

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.

Giraffes sleep on their rumps

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

Dolphins sleep with half their brains turned off

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.

Bullfrogs might not sleep at all

A 1967 study found that bullfrogs don’t sleep. The experiment found that “even during the resting phase they never failed to show a change in respiratory responses after painful stimuli (cutaneous shock).”

The dormouse can hibernate for 11 months

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 can sleep for up to 18 hours a day

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.

Large dogs dream longer

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.

Fish sleep in suspended animation

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.

This article is an excerpt of a story published by news website Stacker shared under a Creative Commons License.