sometimes I can crush a piece of music with my will, but at other times even if I have practiced a piece too perfection If I dont feel "right" then I dont feel like I am doing the piece justice (to me it is like a mental energy thing) if I am (drained, or charged)
If you've had 2 hours of sleep, still recovering from the flu and have a blocked up nose, there's no way in Hell you will be able to pull the piece of. You have to be in a very good mental state of mind.
Don't tell Keith Jarrett that. Those were pretty much his conditions (if not worse), the day he gave a concert in Germany in the 70s. Today that very concert is known as "The Koln Concert", arguably one of the most magical, inspired musical achievements in the history of recorded music.