根据短文内容，从短文后的选项中选出能填入空白处的最佳答案，选项中有两项为多余选项。
It's such a common event that you probably never asked yourself why you sleep. 36 .In fact, for sleep researchers it's one of the biggest unanswered questions in the field.
Stop and think about it for a second. Why should we sleep? 37 .But we sleep every night even when we have had plenty of rest. There are, no doubt, several different answers to this question but let's just consider one general purpose of sleep: 38 . Human beings are creatures that are normally active during daylight hours when our senses functions most effectively. 39 .We can't see objects well, our color vision is entirely lost, and we don't have the smelling or hearing sharpness of other animals. So it actually does make sense to have us stay where we are during the dangerous period when night-waking animals are walking here and there. And one sure way to make sure we don't fall down everywhere and get lost or eaten is to have us not move 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. 40 . Perhaps even the most important one.
A. At night, humans do rather poorly.
B. But from a scientific point of view this is far from an ordinary matter.
C. Should that put human beings in a terrible situation?
D. Sleep as a survival approach.
E. Furthermore, if you were designing an animal would you have it come into long periods of unconsciousness every twenty-four hours?
F. But in terms of evolution it may have been one of the first reasons.
G. Is it because we get tired?
