I did the exact same while working full time and am trying to do the same as a freelancer now.

What I've found is that no matter how early I wake up I have about eight hours in me and then I lose the ability to focus. I have a feeling exercise and a better diet might help.

Don't get me wrong though, having an ultra productive 4 hours before 10am is awesome, and clocking off at 14:00 to read, play with my arduino or do other things is pretty good too. For me at least though it hasn't magically added any extra hard-focus time to the day.