When you nap, you often stay in one position and donโt move much, so your skin gets compressed against fabric or folds in your clothes. That pressure restricts blood flow for a short time, leaving temporary impressions or marks (like pillow lines or wrinkles).
When you sleep for a long time, though, your body naturally shifts positions throughout the night, dozens of times which evens out the pressure. By the time you wake up, any prints that mightโve formed earlier have already faded.