Endorphins.  Many of us do those things that are considered a challenge often because subconsciously we know a feeling of well being and euphoria will follow.  There is much written about physical exercise and the morphine-like effect of the release of endorphins by the central nervous system and pituitary gland.  Recently, more has been written about similar affects of exercising the brain when writing.  I know that when I have successfully penned a few pages, I do experience that good feeling felt after a hearty physical exercise.  That said, I typically will not feel inclined to write when I am less than chipper.  So, one could argue that to have a successful write, maybe one needs to exercise first.  The overall impact will be a double high of sorts.  But wait, what if you are not up for exercise?  This may be a chicken and egg dilemma.