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Michael: Sleep. We all need it, and we spend about a third of our lives doing it. Scientists aren’t totally sure why we need to sleep, though they do have a few good guesses. It’s obviously an important biological function, because practically every single animal needs to sleep in some form or another. But since it isn’t always practical for some species to just conk out for 8 hours, a lot of animals have evolved some clever workarounds.

[1. Sleeping Standing Up] Large animals like horses, elephants, and rhinos can be pretty vulnerable. They live on the open plains, and they’re so big that they’re very visible. They have to be prepared to run basically whenever, which makes it dangerous for them to lay down to sleep. So, many of these bigger herbivores have developed mechanisms that allow them to doze on their feet, thanks to something called a stay apparatus.

That’s a system of ligaments and tendons that can lock into place so they don’t have to actively use their muscles to stand upright. Some species, like horses, do need to lay down for REM sleep, the deepest kind of sleep. This is where being a herd animal comes in handy.

While a few animals are lying down to catch some REM sleep, others are sleeping on their feet. By distributing the two kinds of sleep across the herd, large herbivores can guarantee that the group is ready to react to danger, but its members can also take turns getting deep sleep.

[2. Sleeping Very Little] Like horses and other large herbivores, giraffes sleep standing up. But that’s not what makes them so interesting. As it turns out, giraffes get very little sleep.

Researchers observing giraffes in captivity found that adult giraffes get less than five hours of sleep per day. Like horses, giraffes will sometimes lay down to catch some a little deep sleep, twisting their neck around to rest it on their rump like a pillow. But they only do this for a few minutes at a time, and less often once they’ve reached adulthood.

Researchers aren’t sure how these huge animals can get away with such short naps, but it’s probably an evolutionary advantage. Sleeping less means they spend more time alert and aware of their surroundings and less time unconscious and at risk of being attacked by predators. It could also be necessary. Since giraffes are both vegetarians and enormous, they have to eat a lot — up to 37 kilograms of leaves per day. And it takes a long time to eat that much food!

[3. Sleeping in Flight] Frigatebirds lead pretty intense lives. Like some other birds — swifts, for example — they can fly for weeks without stopping. And they can sleep at the same time! Flying uses a lot of energy. So for a long time, ornithologists figured that these birds had to be sleeping somehow.

But they didn’t know much about the specifics. In a study published in August in the journal Nature, researchers captured 15 frigatebirds and attached sensors to their heads, then released them. They monitored the birds’ brainwaves as they flew, taking trips that lasted up to 10 days and covered distances up to 3,000 kilometers.

The team found that the birds did sleep, even though they were flying. They didn’t sleep as much as they normally would on land, but still — they slept. In the air. So it’s possible that other birds that fly for extended periods of time also sleep in flight.

[4. Uni-hemispheric Sleep] Other animals can’t really afford to let their whole brain sleep at once — like marine mammals that live underwater. These mammals have to deal with a tricky situation: they need to breathe air. Which means that they need to do two things: First, they have to consciously control their breathing, so they don’t accidentally inhale water. They also have to rise to the surface to breathe regularly, which seems like it would make sleeping nearly impossible.

Bottlenose dolphins have a solution to this problem: only half of their brain sleeps at a time. That’s uni-hemispheric sleep. Meanwhile, the other half of the brain maintains some awareness, staying alert for danger and allowing the dolphin to surface for air every 4-5 minutes.

After about two hours, the dolphin switches hemispheres, so both get the chance to rest. And dolphins aren’t the only creatures to use this method of sleeping. Ducks do it too! Mallard ducks sleep all lined up in a row, and ducks in the middle of the row will close both eyes and go completely to sleep. But the ducks on the ends of the row keep their outside eye open, and half of their brains awake to keep watch.

[5. Drift Diving] Marine biologists used to assume that pretty much all mammals that lived underwater used the same half-brain method to handle breathing and sleeping. Then, in 2008, they found out that sperm whales might actually have a different way of making sure they get their rest. See, sperm whales can hold their breath for a really long time — up to 90 minutes by some estimates. So unlike dolphins, which need to breathe every few minutes, sperm whales can go for a while without surfacing.

Which might be why they seem to have an usual napping technique. A group of researchers came across a pod of sperm whales just... floating vertically in the ocean. These whales were completely motionless, and didn’t react to the researchers’ presence until their boat accidentally bumped a whale.

The scientists were curious about why the whales were drifting around vertically in the ocean, so they decided to investigate. By using tracking tags, they found that sperm whales consistently do these so-called drift dives, and will spend up to 30 minutes motionless and vertical in the water, before coming up to breathe again. The whales are so motionless and unresponsive that they’re probably in some kind of sleep state.

But their overall time in these dives is low — less 3 hours per day, which probably wouldn’t be enough sleep on its own. So it’s likely that sperm whales also use uni-hemispheric sleep. But these drift dives might be an opportunity for deeper, more restful sleep.

[6. Yo-Yo Diving] Other swimmers can’t stay still while they sleep. Like some sharks, for example. It’s not clear whether sharks sleep in the way we typically think of sleep, but they do have some kind of resting period. But some species of shark have to keep moving so that water can flow across their gills, which means they’d have to swim in their sleep to stay alive. So they sleep using a technique that marine biologists call yo-yo diving.

Basically, they swim up toward the surface, then allow themselves to glide downward for a while. They rest as the water moves across their gills, then wake up and repeat the process. This specialized diving might be enough to let sharks get the rest they need while still letting them breathe.

This isn’t true for all species of sharks, though. Some sharks don’t need to be constantly moving because they’ve evolved spiracles. These specialized gills behind their eyes draw in oxygenated water, even while the sharks are laying on the ocean floor.

[7. Sleeping Inside Snot Bubbles] Some species of parrotfish have kind of a gross sleeping habit: When they settle down for the night, they blow a big snot bubble using special glands near their gills and cuddle up inside. It takes a long time and a lot of energy to make these mucus sleeping bags, so they must be important for the parrot fish — but why? Researchers used to think that the bubbles prevented silt from settling on the fish during sleep, or helped keep them hidden or warned them of approaching predators But there wasn’t good evidence to support any of these hypotheses.

Finally a few years ago, some scientists found out that bullethead parrotfish protected by cocoons were less likely to be attacked by a nasty blood-sucking parasite called a gnathiid. See, during the daytime, parrotfish can seek out cleaner fish, like the Bluestreak cleaner wrasse, to take care of the gnathiid infestations. But at night, the wrasse aren’t around to help out. So it’s possible that these parasites are so damaging to parrotfish that they evolved these mucus bubbles to keep them out while they’re sleeping!

[8. Invertebrate REM Sleep] Octopuses may be a lot more like humans than we thought. Because they seem to experience REM sleep, just like us. Since they’re invertebrates and their physiology is so different from our own, researchers used to assume that octopuses didn’t really sleep in a technical sense. Many invertebrates have periods of wakefulness and rest, but their brains are so different from vertebrate brains that it’s difficult for scientists to figure out what, exactly, would be considered sleep for them.

Then marine biologists noticed that cuttlefish would lie still with their skin a dull color for periods of 10-15 minutes, then would briefly twitch and flash different colors, then repeat the cycle. Further research showed that twitching and random color changing in octopuses was a sign that they’d entered into rapid eye movement, or REM, sleep. REM sleep is important for learning in mammals, and if some invertebrates do it too, it might be connected to their learning process, as well. It also might help explain why these invertebrates seem to be unusually intelligent. Like the other animals on this list, they’ve evolved get just the kind of shut-eye they need.

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