It's just a cat thing. They like to mess with the people they really like while they're sleeping. My mom's cat does the same thing only he knocks the rollers out of her hair. Sammy gets away with murder.
If it is the eyes, it wants you to wake up - probably for food. If it is the mouth, you are snoring too loud - it is not your normal "noise" and the cat is concerned. Anywhere else usually means it is usually just playing - or defending you against the alien invaders trying to take control of your body.