By Jon Stewart, BBC Science

媒体英语会带大家一起学习BBC撰稿人在报道世界大事时常用到的单词和短语。

Background: 一项新研究对长期未解的一个科学问题提供了答案，这就是睡眠究竟有什么意义。科学家原来以为睡眠对于大脑修复，保持长期记忆力和扭转清醒时候压力过渡导致的损害都有意义。然而最新的研究之出睡眠主要的作用在于让各种动物更好地适应生存环境。

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, conducteda 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.

• background [´bækgraund]   n.背景；经历；幕后   (初中英语单词)
• function [´fʌŋkʃən]   n.机能；职责 vi.活动   (初中英语单词)
• california [,kæli´fɔ:njə]   n.加利福尼亚   (初中英语单词)
• survive [sə´vaiv]   vt.幸存；残存   (初中英语单词)
• sleeping [´sli:piŋ]   n.&a.睡着(的)   (初中英语单词)
• conserve [kən´sə:v]   vt.保存 n.密饯   (初中英语单词)
• energy [´enədʒi]   n.活力，精力；能力   (初中英语单词)
• otherwise [´ʌðəwaiz]   ad.另外 conj.否则   (初中英语单词)
• efficient [i´fiʃənt]   a.有效的，有能力的   (高中英语单词)
• limited [´limitid]   a.有限(制)的   (高中英语单词)