But sleep is a natural need, and, like any other natural need, varies in degree in different persons. Dogs, cats and other animals generally sleep more than we do, and their young ones sleep still more. Generally speaking, the infant, whose mental powers have barely awakened, who is, so far as we can tell, merely a human animal, needs more sleep than it will ever need again in its existence. In this great need of sleep the human animal resembles other animals. It frequently happens that, as a man waxes older, he requires less and less sleep than in his growing and most active years. But old people who have outlived their mental life come to a time when they sleep and perform merely the physical functions like the infant; so also with those whose energy so far exceeds their physical strength that the mere effort of living exhausts them. This condition may be in part due to overstrain of the powers of youth and middle age, but it also follows the fixed idea that years diminish strength and lessen energy. It is easy to fall into this notion, for it accords so well with the general idea that rest must come only after the period of activity, whether that period be a day or a lifetime. All of us have had periods when we have needed fewer than our average hours of sleep. People who sleep out of doors or in thoroughly ventilated rooms, under warm but light clothing, find that they need less sleep than when they occupy poorly ventilated rooms and wrap themselves in heavy, unhygienic clothing. Fresh discoveries are being made almost daily by those who give intelligent consideration to these things. Even babies differ in their need of sleep. I know one healthy, happy, beautiful baby who has never slept the average sixteen hours that babies are supposed to need. This child is now between three and four years of age, and has never gone to sleep before nine or half-past nine at night. Her parents had the common idea of long hours of sleep for infants, and the child had a hard struggle for a while to convince them that she had no such need: such struggles are often called “naughtiness.” She was regularly put to bed at seven o’clock, and all the usual devices for enticing a baby to sleep were practiced. Sometimes she was left severely alone, sometimes she had gentle lullabies sung to her, but, whether alone or in company, this particular baby played and enjoyed herself until between nine and nine-thirty, when she quietly dropped to sleep. She awoke as early as the average baby wakes, happy and refreshed, and her parents finally learned that there is no sleeping rule that has no exceptions, whether applied to infants or adults.