I've never found that my most productive work periods are limited to the standard eight and five work day.
As a result, I work at odd hours and on a project by project basis. Probably 96 percent of my evenings involve some sort of work. Early morning is another productive time period.
If a brick wall is hit on one project, I move to another. Staring at the wall usually doesn't revive my focus but leaping into a different area eventually renews enough interest that focusing on the first project is again possible.
As mentioned in other posts, intense, productive work tends to come in bursts rather than in one, continuous flow. This means that the day is often devoted either to working in bursts, recovering from the bursts, or preparing for them. This approach is possible due to the nature of my work. In another line - factory work comes to mind - it would not be possible.
I wonder though, if the "working in bursts" pattern isn't natural for most individuals. Does this match your natural schedule (versus the required one)?