【英语时差8，16】深处睡眠1

 Almost every animal ever studied spends a large part of each day sleeping. Sleep is thought to play an essential role in the health of almost all animals.   Because of this, scientists have long wondered how cetaceans, such as whales and dolphins, get enough sleep. Since cetaceans must surface regularly to breathe, if they drift off to slumberland at the bottom of the sea they could drown. And although whales and dolphins are sometimes observed resting for short times at the surface, others are known to swim continuously for weeks without stopping.

Is it possible that cetaceans have evolved the ability to survive without sleep?

A group of Russian scientists didn't think so. They had noticed that bottlenose dolphins sometimes swam in slow stereotyped¹ patterns with one eye closed. They investigated the behavior by measuring the dolphins' brain-wave activity.

They discovered that half of the dolphin brain, the half controlling the closed eye, was apparently² fast asleep, while the other side of the brain, the side controlling the open eye remained awake and alert!

By keeping one half of their brain alert and awake while the other half sleeps, dolphins can literally³ "keep an eye out" for danger or obstacles in the water, keep in contact with their group, and surface to breathe.

This kind of sleep, called uni-hemispheric slow wave sleep, has now been discovered in several more species of whales and dolphins. So while cetaceans aren't totally sleepless⁴ in "sea"-attle they have evolved an extraordinarily⁵ different way to sleep than their land-loving relatives.   

1 stereotyped   

adj.(指形象、思想、人物等)模式化的

参考例句：

• There is a sameness about all these tales. They're so stereotyped -- all about talented scholars and lovely ladies. 这些书就是一套子，左不过是些才子佳人，最没趣儿。
• He is the stereotyped monster of the horror films and the adventure books, and an obvious (though not perhaps strictly scientific) link with our ancestral past. 它们是恐怖电影和惊险小说中的老一套的怪物，并且与我们的祖先有着明显的（虽然可能没有科学的）联系。

2 apparently   

adv.显然地；表面上，似乎

参考例句：

• An apparently blind alley leads suddenly into an open space.山穷水尽，豁然开朗。
• He was apparently much surprised at the news.他对那个消息显然感到十分惊异。

3 literally   

adv.照字面意义，逐字地；确实

参考例句：

• He translated the passage literally.他逐字逐句地翻译这段文字。
• Sometimes she would not sit down till she was literally faint.有时候，她不走到真正要昏厥了，决不肯坐下来。

4 sleepless   

adj.不睡眠的，睡不著的，不休息的

参考例句：

• The situation gave her many sleepless nights.这种情况害她一连好多天睡不好觉。
• One evening I heard a tale that rendered me sleepless for nights.一天晚上，我听说了一个传闻，把我搞得一连几夜都不能入睡。

5 extraordinarily   

adv.格外地；极端地

参考例句：

• She is an extraordinarily beautiful girl.她是个美丽非凡的姑娘。
• The sea was extraordinarily calm that morning.那天清晨，大海出奇地宁静。