Several times a day, get up and move around and focus on absolutely nothing. If you can get outside, walk through grass or through the parking lot looking for an odd rock or a lost coin...but just let your mind get away from serious matters for 5 or 10 minutes and take a mind break.
Focusing too long and too hard on a subject or task can cause mental exhaustion. We all need time to just de-compress and a few minutes in mindless activities can be like a mini-vacation. Take deep breaths, stretch, wiggle, rest your eyes...but do something besides intense focusing!!!