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 his 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 rismg 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, lke 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 [’am drowning or falling, I wake 
in agony before the dream is ended. An unusual noise, a sudden 
gleam of light, 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. 
