If Stress Leads To Depression,
Why Isn’t Everyone Depressed?
There is a strong link between stress and depression.
And the brain’s runaway stress response has been found to be a key trigger for depression.
When stress leads to depression
Stress could arise from traumatic events, like the loss of a loved one, divorce, experience of abuse, accidents or disasters, etc. Stress could be in the form of daily modern life hassles, such as being stuck in a traffic jam while running late for an appointment, facing a tight deadline at work, worrying about finances etc.
Stress could arise from emotional conflicts (e.g. feelings of guilt, resentment etc), or conflicts with significant others (e.g. quarrels with spouse or parents). Stress could also be brought about by internal conditions such as illnesses or pain.
Even positive events such as a new marriage or a job promotion can be stress-provoking.
Given that stress is such an unavoidable part of life, why then do some people develop full blown medical depression, while others get away with just suffering from low moods for a while?
The critical factor in triggering depression, it seems, is not the presence of stress per se, but an over-run of the body’s stress response .
The body’s response to stress
When we are faced with stressors, adaptive systems in our brain and body are activated to prepare us to rise to the occasion.
For example, hormones such as adrenaline and cortical are released, triggering a host of other reactions. The liver converts its store of glycogen into glucose that can be used directly by the body as fuel. The lungs increase their intake of oxygen, which is needed in greater quantity when the muscles are in action. The heart beats faster and stronger so as to send the nutrients and oxygen rich blood to all parts of the body. 
The immune system also shifts its focus to tissue repair (instead of attacking invaders) in preparation for any physical injury that may arise. For stressors that extend into the night, the brain is prompted to change the structure of sleep from the deep restorative slumber to the shallower, restless and dream-filled sleep that is less healthy for the body, but is nonetheless crucial to help us wake up more readily in the event of danger. 
Once the stressor or threat is removed, the body quickly shuts down its stress circuitry, the body is restored to normalcy and levels of circulating stress hormones are kept at a low. 
How stress leads to depression
As you can see, most of the body’s reactions to stress are physical responses and are aimed at preparing us quickly for an intense burst of action (i.e. the fight-flight response), more suited for the sort of intense, short-term and physical challenges that our hunter-gatherer ancestors faced (e.g. running away from a predator). 
In the face of the more chronic and less physical daily hassles and pressures that our modern life pose to us today, our body’s stress response system may seem more ancient .
Though the stress response may help the individual cope with the modern day stressors in the short term, its mal-adaptation to chronic stress may in fact bring more harm (e.g. increased susceptibility to illnesses, hormonal imbalances etc) than good .
While the modern everyday (chronic) stressors can predispose us to developing depression, fortunately, they usually do not trigger depression on their own. 
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