From “Getting Relief From Light Therapy” in the Fall 2007 Johns Hopkins Depression and Anxiety Bulletin:

After years of lurking on the sidelines of depression treatment, therapy with bright light is finally winning a more prominent place in the arsenal of depression fighters. A major research review, commissioned by the American Psychiatric Association, recently concluded that light therapy effectively treats mood disorders, including winter depression [SAD] and other forms of depression.
Researchers took a hard look at 173 published studies of bright-light therapy for depression. Most of the studies were poorly designed and excluded from the review, with only 20 studies passing muster. But even after the flawed studies were excluded, the remaining pooled data clearly showed that bright-light therapy yielded substantial relief both for SAD and mild-to-moderate depression that was not season-specific. The beneficial effect of the light therapy was comparable in strength to that of conventional medications used to treat depression.
The bottom line, the researchers say: Bright-light therapy is as effective as antidepressant medication [IMPORTANT CLAUSE HERE] for mild to moderate depression.