Finally our mental activity does not cease during sleep. Wake up a sleeper at any time and he will awaken _from a dream_. He may not be able to tell that dream but he will know for sure that, not only was he dreaming, but had been dreaming for a long while before awaking. Wherein, then, does sleep differ from waking life? Solely in the form of our mental activities. Sleep is not as Manacéine, the author of the most complete book on sleep, stated: the resting time of consciousness. We do not withdraw our attention completely from the environment in sleep. When we make up our minds, for instance, to wake up at a certain time, we seldom fail to carry out our purpose. Which does not mean that we are suddenly aroused out of our unconsciousness by something within ourselves, but more probably that our attention has been concentrated all night on certain stimuli indicating time, distant chimes, activities taking place at a definite hour, and which we had noticed unconsciously, although they may have escaped our conscious attention. It has even been suggested that as respiration and pulse are more or less constant in rest, they are used by the organism as unconscious time-registers. This is possibly one of the phenomena due to the activity of the pituitary body in which may reside the "sense of time" and which controls all the rhythms of the body. Jouffroy, Manacéine and Kempf have remarked that nursing mothers may sleep soundly in spite of the disturbances which take place about them, but that the slightest motion of their infant will awaken them. Many nurses not only can wake up at regular intervals to administer a drug to their patients, but, besides, can be aroused out of a sound sleep by a change in the patient's breathing foreboding some danger. Our withdrawal of attention from reality follows the same curve as that followed by the withdrawal of blood from the brain. Many experiments have been made to determine that curve and to sound the depth of sleep. In one case a metallic ball was allowed to fall from varying heights until the noise awakened the sleeper; in another case electric currents of varying voltage were used to stimulate the subject, etc. All experiments have yielded the same results: Sleep reaches its lowest depth during the first two or three hours, _the average time being shorter during the day than at night_. In the majority of subjects, the greatest depth is reached about the end of the first hour. After the third hour, sleep is easily disturbed, the more so as the usual awakening time approaches. To conclude, we will say that sleep partakes of all the characteristics of normal life, the only essential difference we can establish scientifically being a greater withdrawal of attention from reality in normal sleep than in normal waking life. We insist on using the terms _normal waking life_, for there are forms of abnormal waking life in which attention is withdrawn as completely from reality as it is in normal sleep.