Fatigue isn't just something that happens to your quads when you finally make it in to the gym to work on that resolution. Working on the same task for too long without a break can send you into a mental fog, but there probably (hopefully) won't be any sharp burning to tell you it's time for a break. If you just can't seem to get the sound you want out of a mix, try taking a break. Take a walk around the house, have a cup of tea, or go vent your frustrations to your pet. You might be surprised what an impact a short break can have on your ability to get work done.
You can read more here: http://www.audio-issues.com/keeping-track/retreat-regroup/