大家都知道，鯨魚和海豚都是哺乳動物，就像人一樣。那么，它們是否也像人一樣睡覺呢？它們是怎么休息的？它們不擔心睡覺的時候被淹死嗎？下面這篇文章將試著為你解開這個謎。

Whales and dolphins are mammals, so in a lot of ways, they are just like human beings. Among other things, they have similar bone structure, are warm-blooded and give birth to live young. The biggest difference between these animals and human beings are related to our respective environments. Whales and dolphins have a unique respiratory system that lets them spend long periods of time underwater, without taking in any oxygen.

Because of their undersea environment, whales and dolphins must be conscious breathers: They have to actively decide when to breathe. This presents a problem, since mammalian brains need to enter an unconscious state from time to time in order to function correctly.

There's plenty of time for a dolphin to catch a catnap between trips to the ocean surface, of course, but this isn't a viable option. When you're a conscious breather, it's just not feasible to be completely unconscious -- what if you don't wake up in time? The solution for whales and dolphins is to let one half of the brain sleep at a time. In this way, the animal is never completely unconscious, but it still gets the rest it needs.

Scientists have studied this phenomenon in dolphins, using electroencephalography. In this process, electrodes hooked up to the head measure electricity levels in the brain. The result demonstrates that in the sleep cycle, half of the dolphin's brain does indeed "shut down" while the other half is still active. Researchers have observed that dolphins are in this state for approximately eight hours a day. It is probably something like the semi-conscious state we experience as we begin to fall asleep.

And where do dolphins and whales sleep? They could probably sleep anywhere, but it makes sense that they would do it near the surface of the ocean so they can come up for air easily.

respiratory: 呼吸的

electroencephalography: 腦電圖

（英語點津 Annabel 編輯）