Optimize Your Time
Most people think that getting the most amount of work done per day is achieved through minimizing the number of distractions you have while at work. While having the right breakdown of time _during_ work is important, far more import is what you're doing when not working.
If you're working 8 hours per day, but only sleep 4 hours per day this is clearly not sustainable.
Society as a whole has decided that 8 hours / day, 5 days per week working and 8 hours / night sleeping is the right mix (with the rest "personal time") This is probably pretty close to right, but if you are fortunate enough to be able to set your own hours, I highly encourage you to experiment with this yourself. Keep track of how many hours you're working, what you're doing when you're not working, and how focused and productive you feel.
The result of this is that you come to realize what you do for those 8 hours not working and not sleeping is super important. This is probably not surprising but the corollary to it is: if you're at work and not feeling productive: STOP. Don't try to keep going and power through it. Creativity doesn't work that way.
Go home. Go jog. Write a snarky blog post about how you feel ;)
You can come back to work later, and when you do you'll be amazed at how much more you can do.