When a person is in stress, the orthosympathetic branch of the autonomic nervous system tends to become hyperactive. This has a number of negative effects. First of all, the orthosympathetic is the branch of the nervous system that expends energy. So, when it's hyperactive, it is often using energy when it is not necessary. If you have tension in your muscles but really don't need to have that tension, your body has to use energy to hold that tension.
People become subconsciously trained to stay in the tension mode.
This hyperactivates the sympathetic nervous system. Also, when the orthosympathetic becomes overcharged, it cancels out the opposite branch in the autonomic nervous system, the parasympathetic, which allows us to relax. The parasympathetic is related to a number of very important things in the body, like regeneration of energy. So, if that system is being inhibited, you're using energy with the orthosympathetic stress response, but not regenerating it though the parasympathetic. The parasympathetic is related to all the positive aspects of digestion, absorption, elimination, and the breaking down of food from the secretion of digestive enzymes.