Here's how Dr. Goesel Anson, plastic surgeon and inventor of the Juverest Pillow, defines sleep wrinkles. "Sleep wrinkles are the lines that are formed when the face is compressed against a pillow night after night and they occur in predictable locations based on fixed anchor points that hold the skin to bone."
It's easy to see sleep wrinkles on a person's face when you know what to look for. Sleep wrinkles are usually vertical and most are on the sides of the face. In comparison, expression lines (the lines you see when you move your face) are often horizontal - with a few exceptions.