Have you been seeing more wrinkles on your face lately? We may have figured out what the problem is. It could be how you’re sleeping. Of course, as you might imagine, stomach sleeping is the worst offender. Sleep expert Dr. Michael Breus says it’s because you’re placing your face directly on the pillow. That causes friction on the skin, stretching collagen fibers, and creating wrinkles. In fact, sleep experts can tell stomach sleepers just by looking at them, because they often have more pronounced forehead lines and pimples.

The position causing the second-most amount of damage is sleeping on your side. Dr. Breus says side sleepers see deeper wrinkles or creases on the side of their face they naturally sleep on each night, as well as a more pronounced eye bag on the side that’s on the pillow, where gravity causes fluid to pool.

If wrinkles are your enemy, the best position for sleep is on your back, face up. Not only will you not get wrinkles caused by pressure and pillow-friction, your face won’t come in contact with all the dirt and oils on your pillowcase, or the grease from your hair, so you’ll be less likely to break out.