PHYSIOLOGY OF SLEEP LEGENDRE. 591 



duration; the newly born slumber 18 to 20 hours a day, adults about 

 8, and the aged only 5 or 6 hours. It varies also in intensity. Some- 

 times we can sleep "with clinched fists," and "a report of a cannon 

 would not awaken us." At other times the least noise wakes us up. 

 Then, certain noises disturb us more than others, and not always by 

 their intensity; just as a sleeping mother is awakened by the slightest 

 movement of her sick child and does not hear other sounds much 

 louder in the street. The soldier in Ms tent is not awakened by the 

 noise of his comrades returning late, but the sound of the bugle wakes 

 him instantly. Clocks strike every hour of the night, but only the 

 customary hour for rising awakens us. A passenger on a steamer 

 sleeping amid the whirr of the propeller may be awakened by the 

 cessation of the sound, so with the miller sleeping amid the clacking 

 of the mill. 



Sleep is not continuous from beginning to end. We have marked 

 its intensity at different periods by observing the strength of irrita- 

 tion necessary to awake the sleeper and we have noted that it becomes 

 deeper and deeper up to the second hour, then gradually diminishes 

 in intensity until the awaking. Waking up, like going to sleep, is 

 premeditated, spontaneous, or provoked. We awaken either because 

 we have wished it, or have slept enough, or because of some exciting 

 cause. Many persons can wake at any hour that they have pre- 

 viously fixed upon, and some never make a mistake of more than a 

 quarter of an hour. In some cases the waking up may be due to 

 habit. Sleep may cease the same hour each day because of a cus- 

 tomary noise, as of the ringing of a bell or the passing of a vehicle, 

 or at the dawn of day. A person may spontaneously end his sleep 

 because he has slept enough, having passed the customary 8 hours 

 in rest. But the best-known causes of waking are external or inter- 

 nal sensations. Hunger, thirst, cold, may awaken one. Disor- 

 dered respiration or circulation, due to an uncomfortable position, 

 produces wakefulness, often accompanied by nightmare. The emo- 

 tion caused by certain dreams breaks our slumber. If I dream that 

 I am going to be crushed or that I am drowning or falling, I wake 

 in agony before the dream is ended. An unusual noise, a sudden 

 gleam of fight, likewise awakens, besides other sensations that I will 

 presently mention. 



In waking up we repass more or less rapidly through the same 

 conditions as in going to sleep. Our consciousness gradually returns, 

 our eyes open, and we remain an instant half awake, all ready to go 

 to sleep again if the cause of the waking should cease ; but if it con- 

 tinues, we take account of our condition, recover consciousness of 

 our surroundings, recognize the time, the light, sounds, and, com- 

 pletely awake, with renewed knowledge of the real, we yield to our 

 first judgment, recover our memory, and decide on our actions. 



