I agree with bluebicycle's advice. I would add one more dimension.

I did my PhD while being treated for bipolar. In my opinion the meds do lead to some fuzzy thinking and I had trouble reading and comprehending anything complex. I was on Depakote when I started and then was switched to lamictal. I didn't notice a difference. It took me longer to finish my dissertation but I did!

My advice is to figure out when you are most productive and do your work then. It may sound obvious but I am serious. I am most productive between 10 and 2pm. That is when I feel the most focused. Hence, I used to do most of my work during those hours. I planned my day around those 4 hours. I realize that may be a luxury or impractical but if you can adjust your schedule around your most productive times you may get some of your focus back.