Because those organs which stimulate the mechanism of the body to act, themselves require rest and repair. When the brain and nerves arrive at that state, they make their condition known to the system generally, by indications which we denominate fatigue.
1018. Why, after rest, do we return invigorated to our labours?
Because the nervous system has accumulated, during the hours of rest, a fresh amount of that vital force which we call the nervous fluid, and by which the various organs of the body are excited to perform the duties assigned to them.
Sleep is understood to be that state of the body in which the relation of the brain to some parts of the body is temporarily suspended.
There are some parts of the body that never sleep: such are the heart, the lungs, the organs of circulation, and those parts of the nervous system that direct their operations.
But when sleep overtakes the system, it seems as if the relations of those parts under the controul of the will were temporarily suspended; as if, for instance, those nerves which move the arms, the legs, the eyes, the tongue, &c., were all at once unfastened, just as the strings of an instrument are relaxed by the turning of a key, or the throwing down of a bridge over which they were stretched.
What is meant by the temporary suspension of the relation of the brain to some parts of the body, may be thus explained. Notice a man when he sits dosing in a chair: at first his head is held up, the brain controlling the muscles of the neck, and keeping the head erect. But drowsiness comes on, the brain begins to withdraw its influence, and the muscles of the neck becoming as it were "unstrung," the head drops down upon the breast. But the sleep is unsound, and disturbed by surrounding noises. The brain is therefore frequently excited to return its influence to the muscles, and draw up the head of the sleeper. He gives a sudden start, every muscle is tightened in an instant, up goes the head, the eyes open, the ears listen, until a feeling of security and composure returns; the sleep again deepens, the nervous connection is again withdrawn, and then down drops the head as before.