The awareness of stress was increased after World War II when it was noticed that many routine life events like examinations, growing up increases stress and leads to dysfunction (Lazarus, 1993). The word stress was used by Hans Selye for the first time to explain the effects of threats that disturbs homeostasis (Chrousos, 1992; Selye, 1978). Walter B. Cannon explained the role of adrenal glands and sympathetic nervous system in maintenance of body equilibrium after exposure to threat. Hans Selye’s observation -” common feeling of being sick” in patients with different diseases laid path for definition of stress. He defined stress as a set of non-specific responses collectively called as” General Adaptation Syndrome”. Hans Selye’s theory of non-specificity was criticized by Mason (Mason, 1971). Gold stein defined stress as “stress is a condition in which expectations, whether genetically programmed, established by prior learning or deduced from circumstances, do not match the current or anticipated perceptions of the internal or external environment and this discrepancy between what is observed or sensed and what is expected or programmed elicits patterned, compensatory responses” (Goldstein and Kopin, 2007).Later the word allostasis was introduced to stress research, which is the process of adaptation to different stressors.When there is any defect in the process of adaptation, it leads to” allostatic load or overload” (McEwen, 1998).

3.2 General adaptation syndrome (Gas)