This paper explains that Norman E. Rosenthal and his colleagues were the first to highlight a pattern of cyclical unipolar
depression linked to the changing of the seasons, most often from fall to winter months through to the springtime, specifically November to March. The author points out three causation theories of SAD: (1) Fluxation of hormone levels, (2) neurobiology related to
circadian rhythms or the body's internal clock, and (3) a biological and hormonal shift dependent on a person's geographical placement or latitude. The paper relates that, while phototherapy with artificial light is an excellent tool for treating SAD sufferers in the winter months, it has not shown to be nearly as effective as natural light, especially light that is absorbed during the summer months.