【英语时差8，16】深处睡眠2

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Although the boat passed within a foot of several whales, they didn't seem to notice or respond. But when the boat accidentally brushed against the side of one whale, it suddenly awoke and swam away, rousing its neighbors.

The biologists decided³ to investigate further. They tracked the behavior of fifty-nine more sperm whales across the world. They found that sperm whales in the wild switch off completely for short periods of time, having "cat naps" while performing slow rhythmic⁴ dives.

Biologists had long thought that whales and other air-breathing marine⁵ mammals survived without any sleep at all. But studies of captive dolphins and small whales later revealed that they did sleep, but only with one side of the brain at a time.

Essentially⁶ one half of their brain shuts off while the other half stays alert, with one eye open. This kind of "uni-hemispheric" sleep allows the animals to swim, breathe, avoid predators⁷ and stay in contact with neighbors while still catching⁸ "forty winks⁹."Although more research is needed to know for sure, sperm whales seem to have evolved a different strategy for sleeping at sea -- sleeping with their whole brain more like terrestrial mammals, rather than sleeping with half a brain and one eye open.

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