'Wake me up in the spring!'

Everyone needs to sleep. Some animals do a special kind of sleep called "hibernation." They stay asleep all winter. Bears, skunks, chipmunks, and some bats hibernate. In the fall, they eat lots of food to get ready, and they get very fat. Their bodies use this fat for energy while they're hibernating. Some animals also store food such as nuts or acorns in fall, to eat later in the winter, if they wake up.

During hibernation, an animal's body temperature drops, and its heartbeat and breathing slow down. Because it uses little energy, the animal doesn't need food. When hibernating animals wake up in the spring, they are very, very hungry.

In everyday life, which animal sleeps the longest? According to experts, the animal that sleeps the most during a 24-hour period is the brown bat, which dozes 19.9 hours every day. The animal that sleeps the least is the giraffe. It snoozes only 1.9 hours daily.

Where do animals usually sleep? Manatees sleep underwater, bobbing to the surface now and then to take a breath. Leopards lounge on the limbs of trees.

Chipmunks, rabbits, and some snakes sleep in underground burrows. Gorillas and chimpanzees make beds out of leaves and branches. Sometimes they get hungry at night and nibble on their "mattress."

Are humans the only animals that sleep on their backs? No. Gorillas and giant pandas also do. A sea otter may sometimes lie on its back to snooze in the water, too.

Not all animals close their eyes when they sleep. Goldfish and other fish have no eyelids, so they can't close them.

No one really knows if sharks sleep. Sharks have been found floating motionless in caves in such places as Japan and Mexico, but when divers moved around, the sharks' eyes followed them.

Dolphins and whales are mammals, so they have to breathe air. That is why they "sleep" close to the water's surface - so they can bob up for a breath.