Yes Clint, in my experience this phenomenon is common to painters. Find I have no awareness of my level of exhaustion until I'm done for the day - only then do I feel the results of standing with my arm extended for hours.

I believe creators have to transcend the thinking mind arriving at a state of stillness in order to receive assistance from sources unknown. It is in this state that clock time ceases to exist and we can soar. It's a magical place.