Cortisol levels in the body follow a circadian rhythm, peaking in the morning and gradually declining until midnight, when it reaches its lowest level. In addition to being influenced by circadian rhythms, cortisol levels are also influenced by exercise intensity, and the body's cortisol levels rise after a hard workout.
Cortisol directs the body to break down sugars, fats and proteins. It either carries fat to muscle in the form of triglycerides or breaks down muscle and converts it into glycogen for energy.
Normal amounts of cortisol help the body recover under stress, while high cortisol levels are bad for muscle growth.

Constant cortisol increases inhibit protein synthesis and speed up protein breakdown, which is detrimental to muscle growth. And it can affect the effects of other anabolic hormones, particularly testosterone. Excessive training can cause high cortisol levels and low testosterone levels, which interfere with muscle growth.
Cortisol reduces the absorption of nutrients into muscles, making it harder for amino acids to reach them and thus harder to grow muscle.
Cortisol also increases myostatin. Myostatin is a protein released by muscles that limits muscle growth. The higher the myostatin level, the harder it is to build a muscle.
Cortisol also causes the differentiation of visceral adipocytes, which increase in number, leading to the growth of visceral fat and central obesity (apple shape).
Cortisol levels rise and water retention occurs. Make the body look puffy.
Elevated cortisol levels affect the growth of osteoblasts and make it harder for bones to absorb calcium, leading to weaker bones.
So it's important to keep cortisol levels within normal limits.