Stress (roughly the opposite of relaxation) is a medical term for a wide range of strong external stimuli, both physiological and psychological, which can cause a physiological response called the general adaptation syndrome, first described in 1936 by Hans Selye in the journal Nature.
Selye was able to separate the physical effects of stress from other physical symptoms suffered by patients through his research. He observed that patients suffered physical effects not caused directly by their disease or by their medical condition.
Selye described the general adaptation syndrome as having three stages:
alarm reaction, where the body detects the external stimuli
adaptation, where the body engages defensive countermeasures against the stressor
exhaustion, where the body begins to run out of defenses
There are two types of stress: eustress (“positive stress”) and distress (“negative stress”), roughly meaning challenge and overload. Both types may be the result of negative or positive events. If a person both wins the lottery and has a beloved relative die on the same day, one event does not cancel the other — both are stressful events. (Note that what causes distress for one person may cause eustress for another, depending upon each individual’s life perception.) When the word stress is used alone, typically it is referring to distress.
Serenity is defined as a state in which an individual is disposition-free or largely free from the negative effects of stress, and in some cultures it is considered a state that can be cultivated by various practices, such as meditation, and other forms of training.
Stress can directly and indirectly contribute to general or specific disorders of body and mind. Stress can have a major impact on the physical functioning of the human body. Such stress raises the level of adrenaline and corticosterone in the body, which in turn increases the heart rate, respiration, and blood pressure and puts more physical stress on bodily organs. Long-term stress can be a contributing factor in heart disease, high blood pressure, stroke and other illnesses.
The Japanese phenomenon of karoshi, or death from overwork, is believed to be due to heart attack and stroke.