Does over-exercising reduce muscle gain?
Will over-exercising affect results when exercising for muscle gain?
Will over-exercising deplete your energy levels and render you unable to exercise to your full potential?
There are a few questions on Fitness.SE that touch on this topic but maybe not from this exact angle. Firstly, overtraining does exist. Stressing your body past its point of recovery can be detrimental, however, as the accepted answer points out most of us are not overtraining. The question then becomes: how do I know if I'm training too hard? Which is asked here and here. Don't confuse over-training with delayed onset muscle soreness. It is OK to train when you're sore.
From a muscle-gain perspective, you need recovery. The rest time is when your muscles actually do the growing that you're looking for. If you check out the importance of rest days you'll find a really nice graphic on how rest periods impact muscle growth.
Repeated links to questions for easier access: