【英语时差8,16】睡觉=休息？（下）

Human beings are “diurnal” creatures. That means we are normally active during daylight hours, when our senses function most effectively. At night, humans do rather poorly: we can’t see objects well, our color vision is entirely¹ lost, and we don’t have the smelling or hearing acuity² of other animals. So it actually does makes sense to have us stay put during the dangerous period when “nocturnal,” or night-waking, animals are on the prowl. And one sure way to make sure we don’t stumble around and get lost or eaten is to have us immobilized for seven or eight hours, rising again only when the light is back and our survival chances are better. It’s not the only reason we sleep; but in terms of evolution, it may have been one of the first reasons — perhaps even the most important one.

1 entirely   

ad.全部地，完整地；完全地，彻底地

参考例句：

• The fire was entirely caused by their neglect of duty. 那场火灾完全是由于他们失职而引起的。
• His life was entirely given up to the educational work. 他的一生统统献给了教育工作。

2 acuity   

n.敏锐，（疾病的）剧烈

参考例句：

• We work on improving visual acuity.我们致力于提高视觉的敏锐度。
• The nurse may also measure visual acuity.护士还可以检查视敏度。