Do animals sleep? Yes. But some do it in ways you could never manage. Bats sleep hanging upside down. Horses can doze while standing up. Sperm whales sleep floating straight up and down.

A bat sleeps about 20 hours a day, and it does it upside down. How does it hang on? Tendons in its feet do the work. They let the bat hang while it is perfectly relaxed. Perching birds have tendons too. In a bird, the tendons lock into place, so it never has to use a muscle to hold on to its branch.

Some animals sleep in the sea. A sea otter wraps itself in kelp so it does not drift away while it sleeps. Fish need to rest, too. A sleeping fish slows down, and its gills pump less. Some parrotfish sleep inside a bubble of clear mucus.

Sperm whales sleep in an odd way: upright. Cameras on tagged whales showed groups of them floating straight up and down, almost motionless. Their heads were at or just below the surface of the sea. They looked like floating pillars.

You may have heard that dolphins never sleep. In fact, a dolphin sleeps with one half of its brain resting while the other half stays awake.

An elephant seal has its own odd way to sleep. On land, it sleeps 10 to 11 hours a day. At sea, it sleeps just 2 hours a day. On deep dives, it takes short naps while it holds its breath.

Some animals sleep a lot, and some sleep very little. A koala normally sleeps or rests for 18 to 22 hours a day. Its food is the likely reason. Eucalyptus leaves have very little energy in them and take a long time to digest. An elephant sleeps only about 2 to 3 hours a day, and a giraffe sleeps mostly at night but takes quick naps through the day.

A sleeping ostrich can fool you. In its deepest kind of sleep, its brain waves are slow and strong. Yet the bird sits still with its eyes open and its neck upright, so it looks alert even though recordings show that it is asleep.

Alpine swifts are birds that may hardly ever land. Scientists tagged six of them with small recorders of light and movement. The data showed the birds stayed in the air for about 200 days, from their trip to Africa through the winter. If they sleep during that time, they must do it while flying. So animals sleep hanging, standing, floating upright, and maybe even in the air.

Words to know

• tendons: foot cords
• mucus: slimy goo
• kelp: sea plant
• upright: straight up

The short version

Yes, animals sleep, but the amount and the way vary a lot. A koala sleeps or rests for 18 to 22 hours a day, while an elephant sleeps only about 2 to 3 hours. Bats sleep hanging upside down, and a dolphin rests one half of its brain at a time.

Quick facts

• A koala normally sleeps or rests for 18 to 22 hours a day.
• An elephant sleeps only about 2 to 3 hours a day.
• Bats sleep hanging upside down, gripping with tendons in their feet.
• A dolphin sleeps with one half of its brain resting while the other half stays awake.
• An elephant seal sleeps 10 to 11 hours a day on land but just 2 hours a day at sea.