Major Depression, Dysthymic Disorder, Seasonal Affective Disorder, and a few other subgroups of major depression are classified as Depressive Disorders.
Everyone can experience periods of sadness or being downhearted, however, these feelings are usually temporary and occur occasionally. Illness, family tragedies, and negative personal experiences are all factors that can contribute to periods of sadness. To be diagnosed as depressed, those feelings of sadness must be persistent for at least 2 weeks.
Persistent depressing feelings can alter the way your brain perceives and interprets everyday events. Depressive disorders can mimic each other in symptoms but differ in length, severity, and frequency.
Major Depression is a persistent, overwhelming, and intrusive feeling of guilt, sadness, disinterest, hopelessness, and worthlessness. It interferes with all daily functioning, can happen a few times in a lifetime, and can last up to a few months at a time.
Dysthymic Disorder has symptoms of major depression that persist most days, every day for more than two years.
Seasonal Affective Disorder is characterized by depressive symptoms that occur seasonally, mostly during fall-winter months.