Finding that sleep usually follows fatigue, and that fatigue is a chemical phenomenon, the so-called chemical theory was next set up, and Sommer was quite sure that sleep comes as a consequence of the exhaustion of the reserve of oxygen in the tissues and the blood, and its replacement by carbonic acid during sleep. But here, too, experimentation has been both inadequate and inconclusive. The vaso-motor theory, as modified by Howell, that sleep is due to the anæmia of the cortical layer of the brain, which invariably takes place when the blood pressure in the arteries at the base of the brain falls, has had a larger and a longer following. But convincing proof is yet to be secured, and Dr. Percy G. Stiles of the Bellevue Hospital ends his discussion of the subject with a guarded inference that there may be truth in both the theories, and that eclecticism is in consequence the wisest policy for the histologist.[1] Footnotes [1] _Popular Science Monthly_, September, 1903. WHAT SLEEP REALLY IS Sleep, however we account for it, is “the resting time of consciousness.”[2] To be sure, there is no absolute arrest of brain activity. There is always, even in the soundest sleep, some cerebral activity.[3] We dream. We have nightmares. We sometimes work out problems in our sleep which have defied our every waking effort. There is on record one instance of a college student who got up at three o’clock to solve successfully, while sound asleep, a problem he could not work out at all before he went to bed. There is another instance well attested of a British consul in Syria who, after tearing up letter after letter which he wrote to a Lebanon emir, went to sleep in sheer despair, only to find when he awoke in the morning, that he had written an elaborate letter which in every way satisfied the multitudinous demands of Arabic diplomacy insistent to the last on all the niceties of Oriental etiquette.[4] Byron was right. Sleep is neither life nor death. It is a world apart. Sleep has its own world, A boundary between the things misnamed Death and Existence; sleep has its own world. Consciousness may be suspended. But the cortical centres are frequently as active when we are asleep as when awake. The attention can be maintained with such unbroken steadiness as to awake some persons with the exactness of an alarm clock on the very minute, even though for purposes of deception the hands of the clock may have been set back without their knowledge. The motor centres can be counted on so confidently that they will drive the somnambulist with the accuracy of a trained chauffeur to his appointed destination. Sleep is, therefore, nothing more than a temporary suspension of a portion of the brain’s activity. FOOTNOTES [2] Manaceïne, 62, 69, 70. [3] Dr. J. Madison Taylor in the _Popular Science Monthly_, September, 1905. [4] Thomson’s _Brain and Personality_, 314. THE NECESSITY OF SLEEP