According to the evolutionary (进化的) theory of sleep, evolution equipped us with a regular pattern of sleeping and waking for the same reason. The theory does not deny (否认) that sleep provides some important restorative functions. It merely says that evolution has programmed us to perform those functions at a time when activity would be inefficient and possibly dangerous. However, sleep protects us only from the sort of trouble we might walk into; it does not protect us from trouble that comes looking for us. So we sleep well when we are in familiar, safe place, but we sleep lightly, if at all, when we fear that bears will nose into the tent.
The evolutionary theory accounts well for differences in sleep among creatures. Why do cats, for instance, sleep so much, while horses sleep so little? Surely cats do not need five times as much repair and restoration as horses do. But cats can afford to have long periods of inactivity because they spend little time eating and are unlikely to be attacked while they sleep. Horses must spend almost all their waking hours eating, because what they eat is very low in energy value. Moreover, they cannot afford to sleep too long or too deeply, because their survival depends on their ability to run away from attackers.
21. The author uses the example of the robot in space exploration to tell us ______.
(A) the differences between robots and men (C) about the need for robots to save power
(B) the reason why men need to sleep (D) about the danger of men working at night
22. Evolution has programmed man to sleep at night chiefly to help him ______.
(A) maintain a regular pattern of life (C) avoid danger and inefficient labor
(B) prevent trouble that comes looking for him (D) restore his bodily functions
23.According to the author, we cannot sleep well when we _____.
(A) are worrying about our safety (C) are in a tent
