Years of pressing one side of your face into a pillow each night causes asymmetric aging. Side-sleepers often see deeper creases on the side they sleep on; stomach-sleepers see the same plus chin creases.
What asymmetric aging looks like
Deeper marionette lines on one side. Vertical creases on the cheek. Under-eye bags that don't symmetrically resolve. Earlier-onset sleep lines that don't fade within an hour of waking.
What helps without changing sleep position
Silk or satin pillowcases (less friction, less creasing). Specialised sleep masks that hold cheeks in position. Wrinkle-prevention pillows (Save My Face, YourFacePillow) suspend face off the surface. Switching to back-sleeping where possible.
Most people can't change their sleep position, but you can dramatically reduce the surface contact damage with a £40 silk pillowcase.