When the stress response system is activated the brain produces a rise in glucocorticoids namely cortisol to pull glucose out of stored tissue.  It prepares the body for the fight or flight survival mechanism to save your life. Glucocorticoids are life saving.  They mobilize energy, increase the cardiovascular system, sharpen cognition through flash bulb memory, you think more clearly.  However, it also shuts down the none essentials, like digestion, a high energy process, reproduction (sex hormones), immunity, tissue repair and growth.  If your preparing for a tornado you can eat a steak and ovulate some other time.  You're facing a short term physical crisis, and the stress response is what you do with your body.  For 99 percent of the species on the planet, stress is 3 minutes of screaming terror in the wilderness, after which either its over with or you're over with.  That's all you ned to know if you're a rabbit or a lion. 

If on the other hand, you're a human then you have to expand the definition of a stressor in a very critical way.  We humans turn on the stress response with memories, emotions, thoughts, poor diets, environmental toxins, caffeine, drugs, and the reality is: That's not what this system was evolved for.  Do it long enough and your going to start to damage every major body system from neurological, to cardiovascular, to carbohydrate, fat and protein metabolism, to endocrine, to musculoskeletal, to eicosanoid (inflammation), to immune, to methylation in the liver.  This is how profound chronic stress reacts with the body systems.  You are more at risk for chronic fatigue, sleep disruption, depression, bi-polar, Alzheimer's, hormonal problems, weight gain, and adult onset diabetes, which soon will surpass cancer and cardiovascular as one of the number one killers in this country.

Now, where no one really associates chronic stress is how it kills neurons in the part of the brain called the hippocampus and weakens the cables between neurons, so they cannot communicate with one another. This impairs the formation and retrieval of long term memory.  The opposite thing happens in the amygdala, the control center for fear.  This area of the brain, the hippocampus and amygdala are the central parts of the limbic brain, the part of the brain that mainly deals with emotions and from those emotions we create flash bulb memory that is later processed into long term potentiation.  

Th limbic part of the brain is where we see all the neurological issues; depression, anxiety, panic attacks, obsessive compulsive disorder, ADD, ADHD, bi-polar, restless leg, PTSD, Alzheimer's, dementia, Parkinson's, insomnia, autism spectrum disorder.  While short term stress is good, it creates memory, long term chronic stress slowly destroys us.  Some of the most well respected neuroscientists have known and studied this for the past thirty years and it is well documented in many scientific journals.  Hippocampus atrophy, a hallmark of Alzheimer's, was first noted back in the sixties.