Many folks I know have gotten used to the idea of working 9 to 5 - eight hours solid. Even those who have the luxury of choosing when they work still stick to this routine. It's something that ends up becoming a habit because most people are usually employees and have to stick to this work schedule. If you have the opportunity to split your work hours it's something I'd recommend doing, or at least giving it a try.
I am fortunate enough to choose when I work and have formed a routine of splitting my work day into two parts - three to four hours of work when I wake up, and another three to four hours before I go to bed. Because I run UltraLinx I have the ability to choose when I work, I'm lucky enough not to be stuck with a strict schedule.
I let my body usually handle when I sleep, how long I sleep and when I wake up. Because I work with some folks in the US I usually end up falling asleep at around 3 AM and end up waking naturally at around 11 AM, so I get a good eight hours of sleep every night. When I wake, I usually get to work as soon as I can, working for around three or four hours.
I will then take quite a large break until my next set of hours of work. You can use this time for other things you enjoy doing. If you love watching films, watch a film. If you're into the gym, go to the gym. Just do something that helps you relax your mind a bit.
I end up getting back to work at around 11 PM and carrying on what I was doing earlier. The break just helps me relax a bit, take a load off and take my time with things.
This may not work for everyone as some people find it hard getting back into work mode, but it's worth giving it a try. It works fantastically for me.