The source of most illness is stress If you often feel tired, no matter how much sleep you get or how much caffeine you drink, if you experience tight muscles, mental fatigue, anxiety, or racing thoughts, stress might be the cause. When the body is stressed, it secretes a hormone called cortisol into the bloodstream. This hormone excites the nervous system, shifting the whole body into a state of readiness and alertness. This is known as our stress response …