I have worked in environments where as the day wore on, my performance plummeted.

After a lot of investigation, I suspect that air quality, lighting, ergonomics can have adverse effects. Only recently I read an article that said that in a room with several persons and poor ventilation the CO2 levels reach levels which are known to impair brain and other neurological functions.

That is, I suspect that your depression is the symptom of a bad environment and not the root cause of your problems.