Here's a little science for you.
Many things happen inside you while you train, one we should take note of is the release of the hormone Cortisol. It is a catabolic (breakdown of tissues) hormone that is secreted when stress (of physical or psychological) disrupts the body's state of homeostasis.
- It acts as anti-flammatory breaks down damaged tissues and ushers them to excretion path
- It promotes fat utilisation for energy
- It counters the effects of Testosterone, a growth hormone responsible for building of tissues
- It is responsible for protein degradation that converts protein into carbs
- It causes the feeling of fatigue and restlessness associated with severe over training
- It depresses the immune function
Cortisol can be a friend or foe so knowing how and when to maintain its levels is a handle to training sustainability. Its levels will rise as training intensity increases so if you are an intermediate to elite level athlete, in order to avoid the symptoms of overtraining an emphasis on strength training mesocycles in your training cycle will allow the cortisol level to return to baseline.