It is not surprising that the relations of psychotherapy to sleep show such a great variety. The factors which cooeperate in normal sleep are many and the disturbance can have very different character. We had to speak of the psychophysics of sleep when we discussed the theoretical relation of sleep to hypnotism and insisted that it is misleading to consider hypnosis simply as partial sleep. We claimed a fundamental difference between the selective inhibition in hypnotism and the general reduction of functions in sleep. To understand sleep, we have to recognize it as one of the fundamental instincts, comparable with the instinct for food or for sexual satisfaction. Every one of such instincts has a circular character. Mental processes, subcortical processes, and physical effects are involved in such a way that each reenforces the others. The physical effect of the sleep instinct, comparable with the pepsin secretion in the food instinct, or with the hyperaemia of the sexual organs in the sexual instinct, is a change in the cortex by which the sensory and motor brain centers are put out of action. What kind of a change that is, is quite indifferent. It may be a chemical one but more probably it is a circulatory one. Let us say it is a contraction of blood-vessels which by the resulting anaemia makes the sensory centers unfit for perception and the motor centers unfit for action. In this way the brain becomes protected by sleep against the demands of the surroundings. The mental reactions are eliminated and the central nervous substance has an opportunity to build itself up. This protective physical activity is now evidently itself controlled by a subcortical center, just as secretion and sexual hyperaemia are controlled. This center probably lies in the medulla oblongata.