Eye boogers, crusties, sleep sand, the sleep in your eyes. No matter what you call it, everyone wakes up with that curious gunk in the corner of their eyes sometimes. And many of us have wondered why it shows up after catching some shut-eye. This video from SciShow has your answer: Eye boogers develop as debris (mostly dust and dead skin cells) mix with the mucus and oil that form the tear film on top of your eye. Blinking usually bats this gunk away, but sleep means less blinking and more buildup in the corner of your eyes.