How do animals catch their zzz's?

Where does a lion sleep? Anywhere it wants.

It's an old joke, but still true. When a lion wants to sleep, it can just flop down on the ground or even hang out in a tree. And male lions especially get plenty of practice, since they sleep as much as 22 hours a day. Other animals have to be a little more careful about where they sleep, so they don't end up as someone's dinner.

How would you like to build a new bed every night? Do you think you could sleep standing on one leg? How would you sleep if you lived underwater but had to come up to the surface to breathe? Different creatures have developed some pretty creative ways to get their rest and stay safe.

The basilisk lizard likes to sleep at the far end of small branches hanging out over a pond or lake in the rain forest. If a snake tries to slither up the branch to eat it, the snake shakes the branch and knocks the lizard off, and it falls safely into the water. Chameleons can change color to match their surroundings in order to hide – even while sleeping.

Gorillas and orangutans like to sleep high in the trees. They build a new nest every night, sometimes taking up to half an hour to pile branches, twigs, and leaves into a comfortable bed. Birds also find it safe to sleep in the trees, but unless they have eggs or young chicks, they don't use a nest.

They just lock their feet around a branch and hang on. A special tendon in their legs is automatically tight when they are at rest, so they won't let go and fall.

Other birds find it safer to sleep on water. Mallard ducks will line themselves up in a row, with the ducks on each end keeping an eye out for danger. Perhaps the strangest way that birds sleep is when they stand on one leg. You may have seen a flamingo sleeping this way at the zoo.

They stand in shallow water, tuck one leg under their bodies, put their heads under their feathers, and go to sleep on one leg like big pink puffballs on sticks. Herons and other birds can also sleep this way. Their bodies are designed so that they can center their balance perfectly over one leg and relax that way.

Water presents its own sleeping problems. Sea otters, which live in the water, float on their backs to sleep. Baby otters may lie on their mothers' stomachs. To keep from floating away, the otters wrap seaweed around their bodies to anchor themselves. Occasionally, a young otter will sleep next to its mother and they will hold paws to stay together.

Dolphins live underwater, but must come to the surface to breathe. Scientists now believe that dolphins may sleep with only half their brain, while the other half stays awake to keep them safe and breathing. Seals also do this, lying on their sides on the surface of the water with one flipper underwater paddling to keep their noses above the surface. Some ducks may also have this ability, and actually sleep with one eye closed and one eye open.

Some animals face a different sleeping challenge. They are just so big that lying down for long isn't comfortable. You may have heard that horses sleep standing up. They can lock their knees and sleep while standing, but the deepest sleep still comes when they lie down. Elephants may lie down and sleep for a few of the coolest hours in the morning, but the weight of their bodies makes this uncomfortable after two or three hours, and they will stand up and nap a little longer on their feet. They also snore.

Even giraffes sleep standing up, and their bodies are balanced so that their long necks support their heads without much effort even in a standing position. Very young giraffes may fold their legs under them to sleep, but still don't lay their heads down.

When animals hibernate, they sleep for a very long time, perhaps even for an entire winter. But some sleep more deeply than others. If you were to find a squirrel hibernating in its nest in a hollow tree, you could probably pick it up and carry it around without waking it.

It might be several hours before the squirrel woke up. But walk into the den of a hibernating bear and he'll know you're there right away and invite you to leave. Animals that hibernate pick places where they aren't likely to be disturbed, such as caves or holes.

Other animals, such as moles, shrews, rabbits, and chipmunks, also find holes a nice place to sleep.

Warthogs, with their long tusks, climb backward into their holes to sleep. That way if an intruder climbs into the hole, the first thing it will find is a pair of tusks facing it.

The octopus sleeps in an underwater cave. Sometimes it will wedge a shell in front of the opening to serve as a door.

Deer find that hiding is a good way to sleep safely. A mother deer will tuck her fawn into a grassy or leafy spot to sleep. The fawn lies still, so as not to give away its position.

Other animals sleep on the ceiling. Bats hang upside down from the tops of caves or from tree branches. Some bats never use their feet for walking, but their curved claws help them to hang from branches or rocks. Sloths sleep hanging upside down in trees, hooking their clawed toes around branches. They sleep about 20 hours a day and may spend their entire lives in the same tree.

Do fish sleep? It's hard to tell, since they never close their eyes. But sharks have been noted to go through resting phases when they slow down their movements. They keep swimming, though, and will notice if prey or predators come near. Other fish also seem to slow down for a period, so they do rest, whether or not we would call it sleep.

Some students feel pretty sure, though, that fish sleep. After all, they travel in schools, and sometimes it can be pretty hard to stay awake in class.

The amount of time animals spend in sleep is related to their size and their metabolic rate, or how fast their body systems work. Here are some average amounts of sleep per day:

2 hours or less: giraffe

3 to 4 hours: elephant, sheep, cow

5 to 6 hours: goat, older human

7 to 8 hours: dolphin, pig, guppy, rabbit, adult human

9 to 10 hours: chimpanzee, squirrel monkey, hedgehog

10 to 11 hours: baboon, mole, duck, jaguar, dog

12 to 13 hours: mouse, cheetah, cat, gerbil

14 to 15 hours: hamster, toad, squirrel

16 to 18 hours: python, opossum, human infant

20 hours or more: sloth, bat, lion

Some animals are nocturnal, which means they are active at night and sleep during the day. These include porcupines, raccoons, toads, weasels, wolves, bats, opossums, owls, and whippoorwills.

Many are diurnal (Die-ER-nal), or active during the day. The third category is crepuscular (kri-PUS-kyoo-ler). These animals are most active at twilight. Rattlesnakes, gila monsters, and other desert animals are crepuscular, which gives them the best access to water. Many rain forest creatures are crepuscular, as are the platypus, rat, and deer.

Bobcats are nocturnal in summer and diurnal in winter. Badgers are nocturnal most of the year, but during the long summer days they will stay awake for part of the day as well.

Some animals also like to take naps. Rabbits take many naps during the day. The ostrich specializes in naps and never sleeps more than 15 minutes at a time. Domestic cats are masters of the "cat nap," and because they don't have to worry about predators, will sleep on and off through day and night.

Sleepy Book, by Charlotte Zolotow HarperCollins, 2001

A Time For Sleeping, by Ron Hirschi Cobblehill Books/Dutton, 1993

Sweet Dreams: How Animals Sleep, by Kimiko Kajikawa Henry Holt, 1999