My conjecture on why it works (for non-SAD cases of depression): People who are depressed are likely socially isolated (by choice, perhaps because of depression, or by circumstances, perhaps causing depression) and also physically isolated in homes or offices without good natural sunlight (again, the physical isolation may be a consequence of depression and wanting to be alone, or causal). I&#x27;ve spent several hours a day outdoors or in buildings (mainly a gym) with large windows providing lots of natural sunlight this year. I&#x27;m also in the best mood I&#x27;ve been in years. Of course, two factors have changed here for me: more light, more physical activity. Both are correlated with improving mood for people with depression, so I can&#x27;t say which has had the greater effect for me.