The concept of stress, as we know it, dates to the mid-1800s, when the English medical journal The Lancet described a form of nervous fatigue resulting from the “incessant shifting of the adaptive apparatus” of perception as objects flashed by the train window. After two world wars and confrontations in Korea and Vietnam, over the course of which it was called “shell shock,” then “combat exhaustion,” “battle stress” and “combat fatigue,” the shock or fatigue involved was understood to be a generalized psycho-neurological ailment, a state of dis-stress or dis-ease, of being unhinged from one's normal basis of equilibrium. By then the stress associated with combat had been studied to the point that preventative action had almost eliminated it. Two factors seemed critical. First, battle stress tends to set in after prolonged, sustained exposure to continuous bombardment and continuous crisis. Second, it also results from a constant experience of the unexpected, of things being beyond one's control and, with this, a growing sense of being overwhelmed and unable to cope. The remedy seemed to be to rotate people away from the front lines before their coping capacity broke down, though this didn't become army policy until after World War II. By that time, too, a third factor had been understood, namely the danger of soldiers being alone and cut off from each other. One key to preventing stress was “the sustaining relationships with others of the battle group.”
To recover to normal cycles after prolonged stress, circadian rhythms need up to five days. Otherwise, stress-hormone levels stay up, even on so-called rest days. The immune system becomes exhausted and endorphins depleted, lowering a person's tolerance for pain and discomfort and increasing their anxiety to the point even of triggering panic attacks. In the words of one researcher, “The mind becomes obsessed with petty problems, and thoughts keep going 'round and 'round like a stuck record.”
Scientists across a range of disciplines have found that stress not only throws the body's pH levels out of balance but causes a rise in cortisol and a drop in salivary immunoglobulin A. Cortisol, which is pumped out of the adrenal gland, is considered the “negative” coping hormone because it causes digestive problems and sleep disturbances.
source: all of the above is from No Time Stress and the Crisis of Modern Life by Heather Menzies