Dysthymia is a more mild type of depression that tends to be chronic in nature with symptoms that are less severe than those of people with major depressive disorders. Bipolar disorder or Manic depression involves alternating episodes of serious mania and depression. The exact mechanism through which depression occurs has yet to be discovered. There are usually a number of factors, both biochemical and psychosocial, that in combination bring about depression. On the other hand it is believed that bipolar disorder is related to an excess of neurotransmitters.
Studies also suggest that stress is a trigger for depression.
Stressors that contribute to depression are defined as major life events and may include things like the death of a spouse, divorce, personal injury, retirement, pregnancy, financial difficulties, change in residence or job. Heredity appears to play a role in depression. It seems that some people may have a brain chemistry that predisposes them to the depressive response, and other people may be at much lower risk of depression even when exposed to the same physical or psychological triggers. No matter how depression begins it is clear that once it starts it quickly progresses into a set of both physical and psychological problems that grow and feed off of each other. Furthermore, early diagnosis and treatment can often reduce the length and severity of depression.