Hi Guys, I call that condition "the fog". Sitting at the computer can definitely wear you out, especially if you are doing a lot of thinking type stuff.
Getting regular exercise sounds obvious but it really works. I find the best thing is to schedule a walk every day - even 15 minutes really helps me to clear my head. The fresh air helps too.
Also, I find it to be like a walking meditation, if I'm stuck on something, many times answers will come to me during my walk.
It gets the blood pumping which feeds your brain and every cell.
It may seem like just one more thing to do, but it really is one of those things that will increase the quality of your work and your health too, which can really suffer if you're spending a lot of time sitting at the computer.
And definitely get up for a stretch of some kind at least every few hours, even if just to walk to the kitchen for a glass of water, another great brain booster.
Over the winter I started drinking more water, now that it's a habit, I am often surprised when it gives me a bigger energy boost than coffee.
I've heard that sometimes when we are tired we are dehydrated.
One more thing
I find that compartmentalizing tasks (I call it using time pockets) helps me because instead of a long line of tasks to complete, I feel finished as I move through each pocket. It reduces the feeling of burn out that comes with a never ending work list.