Truth. Nothing is a greater spurn to creativity (cyclic mental exertion) than time away focusing on cyclic bodily exertion.
The hard part for me is stepping away when I'm grinding on some problem. It always feels like I'm sooo close and this next idea could be the one that lets me walk away victorious.
Usually I'm wrong though and taking a break would be a much better use of my time. Walking, biking, noodling on my guitar, or even going for a drive all seem to work for me.