For muscular mass to increase, three distinct phases must take place. First, growth must be stimulated within the body. That can only be accomplished through subjecting your muscles (and, more specifically, your nervous system) to a high level of muscular output.
The second phase is that of recovery. Both the body and the systems that feed the body must be given time to clean up the metabolic waste products of the workout and to replenish their energy reserves after a draining maximum-overload workout. We’ve recently learned that this process can take anywhere from two or three days to six weeks or more, depending on the level of muscular output employed during the training session and the subject’s innate ability to exercise.
The third and final phase is the growth process itself, which will take place only after the recovery process has run its course. At least one study indicated that the actual growth of muscle may occur in as little as 15 minutes during sleep. However, you must remember that in no case will muscle growth occur until adequate stimulation has been achieved and total recovery complete.
‘Peter Sisco and John Little, Power Factor Training