Some theorists, to be sure, are inclined to think that the fatigued brain cells enter directly through their exhaustion into the protective sleep state. But that simplifies the situation too much. It is quite true, as these theorists claim, that monotonous stimulation of the senses produces sleep. But it is evident that the sleep occurs even then not only in the particular overtired brain cells. A monotonous stimulation of the acoustical center raises the threshold of perception for all the senses and brings sleep to the whole brain. This control of the whole apparatus is thus surely regulated by one definite center. But this lower center, which controls the anaemia of the cortex, is itself directly dependent again upon a mental condition, the mental experience of fatigue. The fatigue sensation, which is possibly the result of toxic processes, works on that lower sleep center, just as the appetizing impression or the sensual images work on the centers of the other two instincts. On the other hand this protective blood-vessel contraction creates again as in the other cases a characteristic organic sensation, the sensation of rest which arises when the threshold of perception and activity is raised. The world begins to appear dim and far away, no impulse for action excites us. This organic feeling of rest associates itself with the fatigue feeling. The fatigue sensation, the subcortical sleep center, the contraction of the vessels in the cortex, and finally the rest sensation form together the complete circle. The difficulty which arises in this case lies only in the fact that the cortex gone to sleep annihilates also, of course, the fatigue sensation and the rest sensation. For that reason the real circle can appear only in the preparatory stages of sleep. As soon as sleep itself sets in, the circle is broken. The circle character of every instinct must lead the physical effect upward to a higher and higher degree. Not to become excessive, the physical effect must be checked somehow. In all other spheres, it finds its end in satisfaction, for instance, by eating or by the sexual act. In sleep the circular process ends automatically by its own effect as soon as complete sleep is reached. Its causes, the fatigue and the rest feeling, are stopped, as soon as the effect, the anaemia, is secured.