Suppose we built a robot (机器人) to explore the planet Mars. We provide the robot with seeing detectors to keep it away from (36) . It is powered entirely by the sun. Should we program the robot to be (37) active at all times No. The robot would be using up (38) at a time when it was not (39) any. So we would probably program it to (40) its activity at night and to wake up at dawn the next morning.
According to the evolutionary (进化的) theory of sleep, evolution equipped us with a regular (41) of sleeping and waking for the same reason. The theory does not deny (否认) that sleep (42) some important restorative functions. It merely says that evolution has programmed us to perform those functions at a time when activity would be (43) and possibly dangerous. However, sleep protects us only from the sort of trouble we might walk into; (44) 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. (45) 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. (46) .
【参考答案】
Moreover, they cannot afford to sleep too long or too deeply......