Everyone has got two personalities - the one that is shown to the world and the other that is secret and real. You don't show your secret personality when you're awake because you can control your behaviour, but when you're asleep, your sleeping position shows the real you. In a normal night, of course, people frequently change their position. The important position is the one that you go to sleep in.
If you go to sleep on your back, you're a very open person. You normally trust people and you are easily influenced by fashion or new ideas. You don't like to upset people, so you never express your real feelings. You're quite shy and you aren't very confident.
If you sleep on your stomach, you are a rather secretive person. You worry a lot and you're always easily upset. You're very stubborn (顽固的) ,but you aren't very ambitious. You usually live for today not for tomorrow. This means that you enjoy having a good time.
If you sleep on curled up, you are probably a very nervous person. You have a low opinion of yourself and so you're often defensive. You're shy and you don't normally like meeting people. You prefer to be on your own. You're easily hurt.
If you sleep on your side, you have usually got a well-balanced personality.