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 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, (46)________________.