Answer: "When you blink, your eyelids push out dried tears, mucus, oils, dirt, and dead cells. When you sleep, you don’t blink, so these substances build up in the corners of your eye and form a crust." @says-nice-toT***PMs

"Sand (also called crust, or gunk) that you can find in your eyes after you wake up is part of your normal bodily functions. This is actually a mix of skin cells, mucus, and oils that are washed away by blinking in the daytime."