When studying the physiology of nerve-action as we see it in animals, children, and men, and the pathology as we see it in various nervous disorders, as acute insanity, 'delirium tremens, and the like, we soon become aware of the fact that the well-being of the entire nervous system depends mainly on its renovation during a state of repose; and that for the higher portion of the brain, at any rate, this state of repose and rest is synonymous with healthy sleep. Round the phenomena of sleep, and its causes and conditions, are grouped many of the problems which have to be solved by the physiologist who has to investigate the action of the nerve-centres, and the physician who has to cure their disorders. The state or condition of a nerve-centre, which I have called the force, will be dependent upon the amount of rest and sleep which it enjoys, supplemented by two other restorers of force, food and warmth; which must also be taken into consideration.
Observation teaches us that all animals sleep after a certain period of bodily fatigue, which varies according to the individual, the young requiring sleep more quickly than the old, and a larger amount. If the fatigue be great, nothing can keep a child or even a man awake. When refreshed by sleep, when the force is again accumulated in the brain, we wake spontaneously, or are awakened by trifling stimuli, as sounds or light. This alternation of sleep and waking is the normal state of health, and absence of sleep is something abnormal: it is a disorder, and must lead to further disorder if prolonged. Sleep is not necessary for the renewal of force in every centre. In very severe muscular exercise mere cessation for a time recruits our force, and enables us to begin again; but for the higher work of the brain sleep is indispensable, and all brain-work, and indeed life itself, must cease, unless by this the force is renewed.
So much does observation teach us of the reparation of the force of the brain during sleep. Experiment, however, enables us to state the physiological condition of the brain in sleep, and so to analyze further the production and expenditure of this nerve-force. In sleep, as we have seen, it is produced and accumulated; in active waking hours it is expended. In sleep, the arterial circulation of the brain falls to a certain point, and metamorphosis consequently is reduced to a minimum. When the brain is acting, even in dreams, the blood-flow increases both in arteries and veins. To promote sleep, we seek to diminish this arterial current; until this is done, sleep comes not. The two things which chiefly produce sleep in a healthy man or animal are fatigue and food. After a hearty meal, or after great fatigue endured for many waking hours, it will be difficult to rouse him from sleep, and when roused he will be torpid and inactive, and will fall back into sleep easily. His brain will be emptied of blood, and ordinary stimuli, as light, sound, and movement of others, will not bring back the blood to his brain: moreover, his blood will contain less
