Welcome to Neurotalk.

I have had persistent problems with photophobia (light sensitivity) and started some threads awhile back discussing some of the strategies I'm using at the computer. Even though you didn't mention light sensitivity, if you're feeling spaced out from looking at the computer too long, it's possible that part of the problem is that the light is overstimulating and fatiguing your brain.

There are a number of simple things you can do to modify your computer to reduce the amount of light you get from the screen. They're easy enough that I think they're at least worth a try in your case. See here:

http://neurotalk.psychcentral.com/thread163832.html

This other thread is probably only helpful if you have visual problems and eye strain, but it might be worth checking:

http://neurotalk.psychcentral.com/thread166767.html

As for nutrition, there are a lot of dos and don'ts. Here is a thread that greenfrog started on "brain friendly nutrition":

http://neurotalk.psychcentral.com/thread165807.html

These are good places to start. If you have more specific questions, please share them.