Even when we are hard at work, we need frequent breathing spells and changes of occupation and amusement to keep one part of our muscles, or our brains, from poisoning itself.
But after a time, in even the strongest and toughest of us, there comes a period when no change of occupation, no mere sitting still, will rest us; we begin to feel drowsy and want to go to sleep.

This means partly that the fatigue poisons, in spite of fresh air and change, have piled up faster than we can burn them, so that we need sleep to restore the body.
All day long we are making more carbon dioxid than the oxygen we breathe in can take care of; while we sleep, the situation is reversed--the oxygen is gaining on the carbon dioxid.

This is why the air in our bedrooms ought to be kept especially pure and fresh.
But the need goes deeper than this: sleeping and waking are simply parts of the great rhythm in which all life beats--a period of work followed by a period of rest.

Continuous, never-ceasing activity for any living thing quickly means death.

While externally the body appears to be at rest, the processes of growth and upbuilding probably go on more rapidly when we are asleep than when we are awake.