By Jon Stewart, BBC Science

媒體英語會帶大家一起學習BBC撰稿人在報道世界大事時常用到的單詞和短語。

Background: 一項新研究對長期未解的一個科學問題提供了答案，這就是睡眠究竟有什么意義。科學家原來以為睡眠對于大腦修復，保持長期記憶力和扭轉清醒時候壓力過渡導致的損害都有意義。然而最新的研究之出睡眠主要的作用在于讓各種動物更好地適應生存環(huán)境。

Pythons, bats and giant armadillos are among the longest sleepers at over 18 hours a day. Human babies need 16 hours, and most of us probably feel we need around eight hours sleep to function well.

Professor Jerry Seigel, from the University of California, Los Angeles, conducted a study of the sleep times of a broad range of animals, and found that they vary widely. Some, like migrating birds, can survive long periods without sleeping at all. He believes that shows sleep evolved to conserve energy:

“It's animals that are needlessly active that will not survive, but animals that are most efficient and use their waking time to do vital functions, and are otherwise asleep that will survive.”

Sleep helps make best use of limited resources. In humans, when we're awake, our brain accounts for 20 percent of the energy we use when just sitting around. Sleeping also makes us less likely to get injured and less likely to be detected by predators.