Sleeping is the one thing that everyone practices almost daily all his life, and that, nevertheless, hardly anyone does as well as when he began. We have improved in our walking, talking, eating, seeing, and in other acts of skill and habit; but, in spite of our experience, few of us have improved in sleeping: the best sleepers only “sleep like a child.” It must be that we do not do it wisely, else we should by this time do it well. Even the race of mankind as a whole does not seem to be able to use sleep, to summon it, or to control it any better than primitive man did. We talk much of the need of sleep, and sagely discuss its benefits, but we know neither how to use the faculty of sleep to the best advantage nor how to cultivate it. Yet for ages men have studied the mystery of sleep. We have acquired many interesting facts concerning its variation, and have formulated a number of theories concerning its cause and advantages; nevertheless, science has given us little real knowledge of sleep, and less mastery over it.