Until you have seen major depression up close, it is hard to appreciate its power. I have seen the lights go out in the faces of people I love. I’ve previously written about how the positive psychology movement, which aims to make people feel happier, is motivating such people into healthier habits. But the fact is, once true depression has set in, "happy talk" falls on deaf ears.

Many people think of depression as an almost catatonic, dysfunctional, suicidal state of mind. I have seen that type of depression invade a person, and I will never forget it. But, much more common, and more insidious, is low-grade depression—the kind that steals your sparkle, and burdens your soul, but does not shout out loudly enough to demand treatment.

In the 2000 book of essays on his work, The Science of Optimism and Hope, Martin Seligman, Ph.D., considered to be the father of the positive psychology movement, wrote about "the remarkable epidemic of depression occurring among young people in the United States today." According to the National Institute of Mental Health (NIMH) approximately 21 million American adults suffer from some form of mood disorder, and 3.3 million of them live with chronic, low-grade depression, which is medically diagnosed as dysthymia.

Though it is sometimes referred to as "mild" depression, mental health experts take dysthymia very seriously. Unfortunately, many people with dysthymia do not. Instead, they struggle laboriously through their lives, toughing out their fatigue, headaches, insomnia, irritability and stress as just a "part of life." What they don't recognize is that their depression may be caused by a chemical imbalance, and it's very unlikely to disappear on its own.

I learned at the 2009 annual meeting of the American Society of Behavioral Medicine that simply treating this type of depression can often relieve many of the other problems people face. Have you battled endlessly with your weight, marital problems or addictions? Do you struggle to control diabetes, asthma or heart disease? Do you suffer from insomnia, fatigue, headaches or other pain? While these things may not be caused by depression, behavioral medicine experts know that they are amplified in people who are depressed.