Sleep positions are completely out of our control, in contrast with gestures, facial expressions, and speech. Therefore this form of analysis, if approached correctly, will give us precise information about someone's personality. You can certainly try to assume the most majestic, to your mind, position while falling asleep (a lotus pose or, for example, a position of a hiding tiger); however, as soon as you fall into the arms of Morpheus, the whole majesty thing will transform into something shapeless and vague.
Do not attempt to deceive yourself; the way how one sleeps is determined by his or her lifestyle. In this case you should analyze precisely the basic position, that, in which predominantly a person sleeps.