A principle only really known by those who really push physical limitations of the human body is that of rest. Rest, surprisingly, is actually just as important as the activity itself. This is called recovery time, but just a fancy way of saying rest.
Without proper rest time, the body does not have time to properly rebuild and adjust to the previous load or strain.
The thing is this is true for the mind as well. You cannot be constantly thinking and straining mentally. To get better you need rest.