Never use totally uneccesary body motions. It completely distracts from the music which is what you want to show.
im playing chopin nocturne op 55 no 1and what i was thinkin to do while i perform it..i can keep my head down in the p parts and in the f parts like waking up??any suggestion or ideas??
Naaa not necessarily. It could be bit like dancing, you connect the mind with your body and strengthen the flow of communication between them. On the other hand, if its planned beforehand like "In that part I will do this and that" then it'll serve no real purpose but to act as a musician, and that's something you shouldn't need to act. If you would want express moods with your body as addition to the actual music, then trust your intuition and don't force it. Movement must come from inside, remember where the master of your body lies - This also guarantees that you will move accordingly to music, not vice versa, thus not distracting you from what really is the source of expression in that performance.
But the thing about "trusting your intuition" on such movements seems suspect. What is good and proper and serving the purpose of music is often that which, at first may not seem natural at all if we're not used to it, and will not come from our intuition. Intuition is based on habits, many of which are often wrong.
It is a common mistake for people to think the expression should be communicated via body movements.