For me, I need breaks, and stimulation. Sometimes, tinkering with fonts and colors is just the right kind of distraction to get a project back on track, and to replenish my joy of coding.

Honestly, I think I have about six hours of full-concentration work in me in a typical day. Less, if it's mentally demanding work. Sometimes, the best thing I can do for a project is take a walk, or let it rest for a bit and let my subconscious keep churning on it.