Usually at some point I start yelling at myself "ok, you REALLY need to work now" and once I get into it I'm extremely focused and productive, especially if a deadline is looming.

When I worked in an office I was the same way. If I hit a good stride by like 4-5pm, I would usually end up staying until 7-9pm. It's good to have bosses that recognize that 40 hours a week don't have to be at the same time every day to maximize productivity.