【B1】 【B9】

According to the evolutionary theory of sleep, evolution equipped us with a regular pattern of sleeping and waking. The theory does not deny that sleep provides some important restorative functions. It merely says that evolution has progammed 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 in a familiar, safe place, but we sleep lightly, ff 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 eats, for instance, sleep so much, while horses sleep so little? Surely eats 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.