All of us know what sleep is. We know how it feels. The meaning of the word is generally not questioned in ordinary conversation and we do not have to look it up in a dictionary. But no one really knows what happens when we sleep. Once we get beyond the simple level of recognizing sleep as a time of rest in which very little activity occurs, we find that most of the questions involving …showed first 75 words of 1529 total…

…showed last 75 words of 1529 total…