Does depressive illness follow a pattern?

Unfortunately, once a person has had a clinical depression, he/she is more likely to suffer from depression again. For example, some people experience seasonal cycles of depression, particularly in winter. This is called Seasonal Affective Disorder (S.A.D.).
Five to ten percent of people who experience depression also experience states of exaggerated happiness or elation called mania. The occurrence of both depression and mania at different times is called bipolar affective disorder, while a repeated experience of depression alone is termed unipolar affective disorder.