In up to 50 percent of people suffering from recurrent episodes of depression, one or both parents also experienced depression. So heredity plays a big role in these disorders. Whatever the factors that trigger it, depression begins with a disturbance in the part of the brain that governs moods. Most people can handle everyday stresses and their bodies re-adjust to these pressures. When stress is to great for a person to handle their adjustment mechanism is unresponsive and depression may be triggered.
Dysthymia is perhaps the most common type of chronic low-grade depression.
This condition involves long-term or recurring depressive symptoms that are not necessarily disabling but keeps a person from functioning normally and interferes with social interactions and enjoyment of life. Research has found that this type of depression often results from unconscious negative thinking. A variation of dysthymia is double depression which a person with chronic, low-grade depression periodically experiences major depressive episodes, then returns to their normal, mildly depressive state. Some people become more depressive in the winter months when the days are shorter. There is usually less activity and the days are darker.