Stress is good for you. It keeps you alert, motivated and primed to respond to danger. As anyone who has faced a work deadline or competed in a sport knows, stress mobilizes the body to respond, improving performance. Yet too much stress, or chronic stress may lead to major depression in susceptible people.

"Like email and email spam, a little stress is good but too much is bad; you'll need to shut down and reboot," says Esther Sternberg, MD, a leading stress researcher and the chief of neuroendocrine immunology and behavior at the National Institute of Mental Health.

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Even positive events, such as getting married or beginning a new job, can be stressful and may lead to an episode of major depression. Yet about 10% of people suffer from depression without the trigger of a stressful event.

The Stress-Depression Connection

Stress -- whether chronic, such as taking care of a parent with Alzheimer's, or acute, such as losing a job or the death of a loved one -- can lead to major depression in susceptible people. Both types of stress lead to overactivity of the body's stress-response mechanism.

Sustained or chronic stress, in particular, leads to elevated hormones such as cortisol, the "stress hormone," and reduced serotonin and other neurotransmitters in the brain, including dopamine, which has been linked to depression. When these chemical systems are working normally, they regulate biological processes like sleep, appetite, energy, and sex drive, and permit expression of normal moods and emotions.

When the stress response fails to shut off and reset after a difficult situation has passed, it can lead to depression in susceptible people.

No one in life escapes event-related stress, such as death of a loved one, a job loss, divorce, a natural disaster such as an earthquake, or even a dramatic dip in your 401(k). A layoff -- an acute stressor -- may lead to chronic stress if a job search is prolonged.

Loss of any type is a major risk factor for depression. Grieving is considered a normal, healthy, response to loss, but if it goes on for too long it can trigger a depression. A serious illness, including depression itself, is considered a chronic stressor.

Stress and Depression: Lifestyle Factors

The connection between stress and depression is complex and circular. People who are stressed often neglect healthy lifestyle practices. They may smoke, drink more than normal, and neglect regular exercise. "Stress, or being stressed out, leads to behaviors and patterns that in turn can lead to a chronic stress burden and increase the risk of major depression," says Bruce McEwen, PhD, author of The End of Stress as We Know It.

Losing a job is not only a blow to self-esteem, but it results in the loss of social contacts that can buffer against depression.

Interestingly, many of the changes in the brain during an episode of depression resemble the effects of severe, prolonged, stress.