Sleep Crust

When we are awake, the natural function of blinking moves the elements of the tear film, including mucus and debris, over the eye and into the punctum. Through the punctum, the tear film drains, and there is turnover of the liquid (and its contents) on the surface of the eye.

When we are asleep, the blink function is essentially gone, so these elements can flow to the corners of the eye. Once they are there, the watery component of the tear film can evaporate, leaving a crusty residue behind.