Here's A very good, but long, read on cortisol and muscle breakdown.

http://www.myosynthesis.com/articles/cortisol-bodybuilding

If you don't want to break a mental sweat reading it all I'll give you a quick synopsis.

Many say you don't want cortisol at all. Why? Well, because it's catabolic and that's bad right? False. High levels of it yes ( i.e. No activity and high stress for a month.) normal levels are good. It helps the breakdown of dead muscle tissue so new tissue can thrive. Cortisol is a stress hormone. High intensity workouts regulate normal levels (especially punching bags, grappling, etc.) high levels break down muscle and reduce protein synthesis in the muscle. After 45min of working out test degrades but cortisol increases meaning workouts are being rendered useless after that point. Not necessarily. Studies showed amino acids with carbs and high levels vitamin c suppress cortisol levels especially when taken during and after a workout. Those who try to limit cortisol completely are hurting themselves. Muscle can't build very efficiently and when it rebounds will be very harmful to the muscles.