Have you heard about anabolic windows? An anabolic window is when you are required to consume the correct amount of carbohydrates and protein within 45 minutes after your workout. According to this theory, if you miss this window, you can say goodbye to your gains.
Researchers found that inadequate post-exercise hydration "significantly increases the major stress hormones cortisol, adrenaline, and noradrenaline as chemicals in the athlete's body react differently when the body is dehydrated.
A body that has just completed a vigorous physical workout can be under stress due to dehydration, causing its core temperature to rise.
Under stress, the body will go to a state of catabolism -- where muscle tissue is broken down as cortisol, a muscle break down hormone, is being released to access the energy it needs to survive the stress. When you stop exercising, your body enters a state of anabolism, that is when it begins to build new muscle tissue.
If your goal is to build muscle, you want to reduce the amount of catabolic hormones in your body, including cortisol, so your body can move quickly from catabolism to anabolism and start building muscle.
The way you do that is by making sure you finish your workout in a fully hydrated state.