Although researchers aren't entirely sure what causes depression, it's believed that environment plays a role along with genetics. And it stands to reason that if an environment is causing repeated stress, a susceptible person may eventually begin feeling depressed. Whether the stress is coming from a difficult relationship, academic pressure, health issues or workplace strife, repeated exposure to upset definitely can take its toll on the psyche.

One of the most common causes of depression is anxiety resulting from work. In fact, a Finnish study of more than 3,300 workers found that those reporting a lack of team spirit in their workplaces experienced a 60 percent higher incidence of depression, and were 50 percent more likely to report being on antidepressants, than employees who felt part of a cohesive, productive unit.  Experts feel that with the U.S. economy shaky right now, the stress of jobs being lost or consolidated, and the agony of watching coworkers leave means more workers are at risk of depression than ever before.

While a definitive biological link between chronic stress and depression has yet to be proven, some scientists feel they're getting closer to an explanation. Researchers at The Rockefeller University in New York have pinpointed a family of proteins known as kainate receptors that they say play a big role in depression. According to scientists at the university, several large-scale studies performed on humans show kainate receptors linked to major depression and suicidal feelings. The researchers performed experiments on rats and found that creating a stressful environment for the rats caused an increase in the production of certain kainate receptors in their brains.

Chronic stress is bad for the body in more ways than just causing or aggravating depression; it's linked to numerous health issues. The best ways to fight it? Making time for yourself every day; learning how to breathe deeply and let go of problems; getting some sunlight and exercise every day; and eating well. A good night's sleep also can be an invaluable tool in fighting the blues that come with stress.

Sources: The association between team climate at work and mental health in the Finnish Health 2000 Study, Occupational and Environmental Medicine, April 2009.

The Rockefeller University,