Think about the last time you felt stressed. Did your heart rate quicken? Did your breathing get shallow and fast? Maybe your muscles tensed, and you became more alert? The brain prompts all these physiological changes to help us survive in the face of a potentially life-threatening situation. But when this response is activated inappropriately or persistently, it can become dangerous. Indeed, research has linked uncontrolled stress to a wide range of health problems, from heart disease and diabetes to depression and post-traumatic stress disorder (PTSD).