Sleep, diet and exercise are most important in my opinion.

I also think varying your activities (not sitting in front of the computer all day), moving around, breaking up your work with some different activities in between. I am a big proponent of doing something different every couple hours even if you have a lot to get done. Can also be a work task but just something different. Staying on the same task for 8-10 hours or more is very draining for me. Need more variety. When I had my business we would work these long projects sometimes to get it all done in a day. 10 or 12 hours straight essentially doing the same thing over and over. I found that much more exhausting than even longer days where I have a variety of different thigs to do. A new task coming up always seemed to keep my mind fresh and didn't notice getting physically or mentally tired.