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Go to Brilliant.org/SciShow to learn more. ♩. Sleep is wonderful, but in the animal kingdom, it's also a surprisingly dangerous activity.

For many animals, lying in one spot, completely motionless, with basically no awareness of their surroundings is just asking for trouble. It also takes away from time that could be spent finding food or other resources. But all animals do it.

And some take it to the extreme. There are some risk-takers who spend almost all of their days sleeping, but they put themselves in harm's way for good reasons. And as a bonus, they're also helping scientists understand why sleep is so useful.

So, here are five animals that sleep way too much — at least, by our standards. The big brown bat is probably best known for its weird sleeping style. Like other bats, it sleeps hanging upside-down from caves, trees, or rock faces, thanks to special tendons in its feet.

But this bat also has one of the longest sleeping times of any mammal. According to some estimates, it gets around 19.7 hours of shut-eye every day. Scientists think the bats' long snoozes might have developed to give them a perfectly efficient and safe daily schedule.

Essentially, sleeping all the time allows them to stay out of trouble and to be awake when their food is. See, these bats are pretty much only active in the few hours around dusk. If they flew around any earlier, birds with keen eyesight and stealthy flying skills could swoop in and make them a meal.

So staying asleep and motionless is actually the safest option. Being active for only a few hours each day also helps bats save energy. Their prey includes things like moths and mayflies, which are also primarily active around dusk.

So even if a bat stayed up late into the night, they wouldn't really get anything for it, since their food wouldn't be around. They'd just be wasting energy. This makes big brown bats a great case for an idea called adaptive inactivity — basically, an adaptation that leads to animals only being awake when they need to be.

Now, another animal that sleeps the day away is the koala. You might have read that koalas sleep for up to 22 hours a day, but that's generally not true. Instead, studies put their total snooze time at around 14.5 hours on average.

Which isn't too bad. Koalas likely sleep so much because their diet of eucalyptus leaves isn't all that good for them. This isn't because these leaves contain toxins, though.

Koalas' digestive systems seem to have adapted to that pretty well. Instead, it's because these leaves don't have many nutrients. That means koalas don't get much energy out of their food, given how much work it takes to digest it.

So sleeping all day is their fuzzy body's way of putting a cap on how much energy they spend. Also, if you're wondering why koalas don't just eat something better for them — well, there aren't many options. As bad as they seem, eucalyptus leaves are actually some of the safer foods in the koalas' environment.

Common opossums are some of the best nappers in the animal world. These ratty-looking marsupials have been recorded sleeping for up to 19.4 hours a day. And that might be because they're born really underdeveloped.

Gestation in opossums only takes about two weeks, and the animals can have up to 20 or so babies at a time. And these joeys are tiny, too, only weighing about a tenth of a gram. That's less than a dime, but it's likely for a good reason.

Most opossums never make it to adulthood, so by having a bunch of huge litters really quickly, an adult can ensure that they have at least some successful offspring. Regardless, these babies spend their first three months or so hanging out in their mom's pouch, basically doing nothing but eating and sleeping until they're grown enough to venture out. Generally speaking, that behavior isn't abnormal for so-called altricial animals — those that are born immature.

Studies have shown that these animals tend to sleep longer and have more rapid eye movement sleep than precocial animals, or those that are born mature. Maybe unsurprisingly, that's because sleeping helps altricial animals put a lot of their energy toward growth — especially rapid eye movement, or REM, sleep. Normally, the opossums would have to put a lot of energy into keeping their bodies warm through thermoregulation, which is a pretty energy-intensive process.

But REM sleeps actually shuts down thermoregulation to some extent, which saves them energy. And since these babies are snuggled up close to their parent or littermates, it's not like they need to make all of their own heat, anyway. These processes are all pretty normal, but opossums take them to the extreme — maybe because they're born extra small and helpless.

As the animals age, they do begin to stay awake for longer, but not by much. And researchers think that behavior is a kind of hangover from their development style. Another altricial animal that you're probably more familiar with is humans.

When we're born, we're pretty helpless and can't do much to take care of ourselves. Scientists are still debating exactly what the advantages to this are, but one idea is that someone can't provide enough energy to a fetus and themselves after about nine months. So, out comes a baby.

To make up for the lost time in the womb, human newborns sleep a lot. They tend to sleep on and off for up to 16 hours a day, and like with opossums, that time is spent putting energy toward growth — especially toward growing the brain. Multiple studies have found that sleep improved memory and learning in human infants, including language learning.

Additionally, two longitudinal studies have linked more sleep as an infant with better impulse control or reasoning a few months or years later. Of course, the tricky thing with these studies is that you can't experimentally change how much sleep babies are getting. That would just be rude.

But since these results have been supported in studies with adults, or even ones with infant rats, they seem reliable. For example, in a 2004 study published in Neuron, 18 adults had their brain activity measured while they slept. Researchers found that areas of the brain's hippocampus involved in learning and memory were active during non-REM sleep, also called slow wave sleep.

And the more active those areas were at night, the better the study's participants did in a memory task the next day. Overall, scientists think that the connections between brain cells might be formed and reinforced as we sleep. And in infants, some of those connections might be forming for the very first time, making sleep super important for how their brain works over the rest of their lives.

In Spanish, “armadillo” means “little armored one”... but maybe they should be called “little sleepy ones”, given that the nine-banded species can sleep for around 18 hours a day. These animals also sleep much more deeply than humans: Around half of their sleep is considered deep sleep, compared to our 5-20%. Scientists think that the reason they can doze off so deeply is because of their distinctive bony plates, called ossified dermal scutes.

These iron-like structures cover most of the top side of their bodies, and protect them if a predator tries to take a chomp. And for added safety, these armadillos also dig underground burrows for sleeping. They don't roll up into little balls, though — only the three-banded armadillo does that.

Still, the nine-banded's anatomy and behavior mean it's protected and can slumber deeply without having to worry about being attacked. That doesn't explain why they sleep so long, though. To figure that out, scientists have had to turn to evolution.

Although we'll need more fossil evidence, there's an emerging hypothesis that the evolution of armadillos' hard shells actually determined a lot of the animals' lifestyle, including their sleeping habits. In a 2001 study published in Evolution, biologists found that mammals with armor often walk relatively slowly. That makes sense because, instead of running away, they have a hard shell to protect from predators.

But also, because they are so slow, it means armored mammals don't need to have high metabolic rates or eat really high-quality foods to give them lots of energy. They can just forage around for the little they need, and spend the rest of the day snoozing. Now, the last five examples really are the sleep exception, not the rule.

Most animals only need a few hours of sleep per night. And even if that still is dangerous, hey, what can you do? Everything needs sleep, right?

Well… maybe not. At least, according to an early 2019 study published in Science Advances. In this paper, researchers tracked the sleep habits of fruit flies.

They observed that 6% of their female flies slept for less than 75 minutes a day, with one of those flies sleeping for only 4 minutes on average — apparently, with no adverse effects. Or at least, without any changes to their lifespan. Everything else is kind of hard to measure in a fly, especially when it's in a lab free from predators.

To dig into this further, the researchers experimentally sleep-deprived some other fruit flies and waited to see how long they lived. For two weeks, they left the flies in a tube that would shake if the insects became inactive for more than 20 seconds. And again, sleep-deprived flies lived about as long as the controls.

This was confusing, because animals have obviously put a lot of time and resources into learning to sleep in safe and efficient ways. But it did lead the scientists to conclude that, while sleep probably serves several biological or evolutionary functions, maybe it isn't always necessary for survival. At least, in fruit flies.

One way or another, this study reminds us that we don't fully understand everything about what sleep does. But if armadillos have their fancy shells, and bats have figured out how to take 19-hour naps… well, there's gotta be at least some reason for doing it. To try and understand why animals need to sleep, scientists have turned to all kinds of methods, including molecular biology.

If you want to learn some of their techniques, you can check out the Order and Information chapter from their Computational Biology course on Brilliant. It's full of great interactive problems and easy-to-understand explanations that make concepts like gene expression easy and interesting to follow. Which is really saying something.

Brilliant has all kinds of courses about topics in science, engineering, computer science, and math, so whether it's molecular biology or astrophysics, there's a lot to learn. They also have Daily Challenges, so you can learn something new in just 5-10 minutes every day. Right now, the first 200 people to sign up at Brilliant.org/SciShow will get 20% off the annual Premium subscription so you can unlock all of their courses and view all the daily challenges in the archives.

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