According to the evolutionary theory of sleep evolution _____1____ us with a regular pattern of sleeping and waking. The theory does not _____2____ that sleep provides some important _____3____ functions. It merely says that _____4____ has programmed us to perform thost functions at a time when activity would be _____5____ and possibly dangerous. However, sleep protects us only from the sort of _____6____ we might walk into; it does not protect us from trouble that comes looking for us. So we sleep well in a _____7____ , safe place, but we sleep lightly, if at all, when we fear that bears will _____8____ into the tent. __________________________________9______________________________________. Why do cats, for instance, sleep so much, while horses sleep so little? ____________________________________10____________________________________. 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, becasue what they eat is very low in energy value.

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