①From tiny ants to giant whales, all animals sleep. Some sleep a little. Some sleep a lot. And some have come up with some unusual ways to get their sleep.
②Koalas live in Australia. They eat only the leaves of eucalyptus （桉树） trees. These leaves are hard to digest （消化） and don't give much energy. So to save their strength, koalas spend most of the day and night sleeping and digesting. They may sleep 18 to 22 hours a day, waking up for short periods to eat leaves. Yawn!Enough talking. Time to sleep!
③Giraffes don't get very much sleep—only a few hours a day. For giraffes, sleeping may mean danger because they may become lions' food while sleeping. So giraffes mostly take short naps （打盹） while standing up. But they do need to lie down for half an hour or so of deep, dreaming sleep. It may take a whole minute for a giraffe to get back up on its long legs again.
④Like giraffes, horses usually nap standing up, but they need to lie down for about 20 minutes to get some deep sleep. When the others lie down, one horse stays awake to keep watch.
⑤Dolphins are air-breathing mammals （哺乳动物） that swim in the sea. Scientists have discovered a surprising fact—these animals can sleep with one half of their brain while the other half stays awake!While the right half of the dolphin's brain is asleep, the left half stays awake!This lets the dolphin keep its nose above the water so it can breathe. After about 20 minutes, the sides switch （转换）, allowing the other half of the dolphin's brain to sleep.
⑥Some far-traveling ocean birds can rest half of their brains at a time while the other half keeps flying. They don't get much sleep this way—when they land, they sleep a lot to make up for it.
