What evidence is there for a correlation between being tightly focused on a problem and only using one part of your brain? I've seen people tightly focused on a problem, in a state of 'flow' and when you break out of it there seems to be a couple of seconds of confusion when they adjust to the circumstances at hand. It is almost as if there is only blood flow, or electrical activity to one part of the brain, and other parts of the brain are dormant. 

Sometimes I see this with people who are doing computer programming and drinking coffee. 

My question is: What evidence is there for a correlation between being tightly focused on a problem and only using one part of your brain?
You may find some explanation from the research done for attention. In the most famous experiment done by Simon and Chabris 1999 (see video), participants inhibited irrelevant information that was not related to the task of counting passes.

When you are performing a task, you pay attention to what is relevant to the task, and inhibit irrelevant internal and external distractions. The more focused you are the more inhibitions occurs. The degree of focus you devote to a task depends from the difficulty of the task and the engagement you have for it.

Programming is a very highly demanding task and usually programmers are very passionate about it. So programming is a very focused task, thus everything unrelated to the task of programming is inhibited. When you do programming, you inhibit information like where you are, who else is in the room, hunger signals (I know programmer who forget to go to lunch) etc. When you are interrupted you need some time to re-activate all those things that have been inhibited.