Environmental factors, however, also play an important role in depression. When combined with a biochemical or genetic predisposition, life stressors (such as relationship problems, financial difficulties, death of a loved one, or medical illness) may cause the disease to manifest itself. John (not his real name), 25, was diagnosed with depression for the first time last year when he and his girlfriend ended their three-year relationship. “I couldn’t do anything because I was totally absorbed with the whole break-up issue,” he says. “It was impossible for me to sleep, and I would wake up at 3 or 4 in the morning and literally shake. And when it was time to wake up, I just couldn’t get out of bed.” In addition, substance abuse and side effects from prescription medication may also lead to a depressive episode. And research shows that people battling serious medical conditions are especially prone to depression. According to the U.S. Department of Health and Human Services, those who have had a heart attack, for example, have a 40 percent chance of being depressed.

Seasonal affective disorder, often called “SAD,” is a striking example of an environmental factor playing a major role in depression. SAD usually starts in late fall, with the decrease in daylight hours and ends in spring when the days get longer. The symptoms of SAD, which include energy loss, increased anxiety, oversleeping, and overeating, may result from a change in the balance of brain chemicals associated with decreased sunlight. The exact reason for the association between light and mood is unknown, but research suggests a connection with the sleep cycle. Several studies have suggested that light therapy, which involves daily exposure to bright fluorescent light, may be an effective treatment for SAD.

Medical professionals generally base a diagnosis of depressive disorder on the presence of certain symptoms listed in the American Psychiatric Association’s Diagnostic and Statistical Manual. The DSM (presently in the fourth edition) lists the following symptoms for depression: depressed mood loss of interest or pleasure in almost all activities changes in appetite or weight disturbed sleep slowed or restless movements fatigue, loss of energy feelings of worthlessness or excessive guilt trouble in thinking, concentrating, or making decisions recurrent thoughts of death or suicide. The diagnosis depends on the number, severity and duration of these symptoms. Even with this list of symptoms, diagnosing depression is not simple. According to the National Alliance for the Mentally Ill, it takes an average of eight years from the onset of depression to get a proper diagnosis. In making a diagnosis, a health professional should also consider the patient’s medical history, the findings of a complete physical exam, and laboratory tests to rule out the possibility of depressive symptoms resulting from another medical problem. The symptoms of the depressive part of bipolar disorder are the same as those expressed in major (unipolar) depression. Because of the similarities in symptoms and the fact that manic episodes usually don’t appear until the mid-20s, some people with bipolar disorder may mistakenly be diagnosed with unipolar depression. This may lead to improper treatment because antidepressants carry the risk of triggering a manic episode.