When you sleep on your tummy, your head is turned to the side. It leads to tension in your neck and shoulder muscles as well as the deterioration of the blood supply to your brain. Dark circles under your eyes and swelling of your face are other unpleasant consequences of such a position. Try to sleep on your back or on your side, but not all the time. The same position can change the shape of your face. Another helpful habit for your skin is choosing the right pillow.