填空题

According to the evolutionary theory of sleep, evolution (36) us with a regular pattern of sleeping and waking. The theory does not (37) that sleep provides some important restorative functions. It merely says that (38) has programmed us to perform those functions at a time when activity would be (39) and. possibly dangerous. However, sleep protects us only from the sort of (40) we might walk into; it does not (41) us from trouble that comes looking for us. So we sleep well in a (42) , safe place, but we sleep lightly, if at all, when we fear that bears will (43) into the tent. (44) . Why do cats, for instance, sleep so much, while horses sleep so little (45) . But cats can afford to have long periods of in activity 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, (46) .