There are a few things that, together, help us to tell whether an animal is asleep. A sleeping animal doesn’t notice everything happening around them. If you disturb a sleeping animal, though, you can still wake them up quickly. Most animals also seem to need a certain amount of sleep every day. If they don’t get enough sleep, they sleep more later (and more deeply) to make up for it. All of this is true for people, too.
Pond snails use things like rocks or the side of their aquarium as their bed, attaching themselves while they sleep. Although this might not seem particularly relaxing, their shells do hang away from their body, and they keep their tentacles inside their shell. Scientists don’t know much about sleep in other snails yet, but they possibly sleep in a similar way.
While most animals sleep like this, relaxed and unmoving, some animals sleep quite differently. Cows stand and chew their cud while they sleep. Ostriches sleep with their eyes open and their heads held high above the ground. Some birds can even fly while they are asleep, and dolphins can sleep while swimming. So even when an animal looks like it’s awake, it might actually be sleeping.
Whales don’t drown when they sleep because, like dolphins, they can swim to the surface of the water while they sleep. People have also seen whales floating just under the water’s surface, which might be another way that whales sleep. They can rest like this for half an hour or more, but they don’t drown because they can hold their breath for much longer than we can.
We know that lots of animals, maybe all animals, sleep. We know that birds, reptiles, frogs and fish sleep, too. Even worms and jellyfish sleep. Because so many different animals sleep, it seems to be very important. We still don’t know why exactly animals first started sleeping, though.
There are also many other things that we don’t know about sleep. Scientists haven’t studied sleep in all animals yet, so we can’t be certain that all animals sleep. Some animals are difficult to study, too. For example, sea sponges are animals, but it would be tricky to tell whether they are sleeping or even awake!
We also don’t know whether other animals dream, like we do. You might have seen pet cats and dogs moving a little while they sleep, as if they are chasing other animals. This might mean that they are dreaming, but again, it is hard to know for sure.
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