Why do you get crust in the corners of your eyes?
You crust in the corners of your eyes because when your eyes close, your eyes still produce tears to keep things moist and lubricated. The excess leaks out the corners and out of the lids dries, causing the crust in your eyes.
Your eyes produce tears even while sleeping. When in REM (rapid eye movement) sleep your eyes move around alot. Tears contain salt which dries when it leaks from the corners of your eyes.
It's a mixture of tears , and the debri your lashes keep out of the eye. It's the same thing that's in your nose.