Nobody wakes up hoping to find crust in the corner of their eye. But that little bit of "sleep" is actually evidence that your eyes have been cleaning and protecting themselves all night.

The medical name for it is rheum. It's a mixture of mucus, oils from glands in your eyelids, dead skin cells, tiny amounts of tears, and microscopic dust and debris your eyes have collected throughout the day.

Normally, you never notice it.

Every time you blink, your eyelids act like miniature windshield wipers, spreading a fresh layer of tears across the surface of your eyes while sweeping away old mucus and debris. That material drains through tiny openings called puncta in the inner corners of your eyelids and into your tear ducts.

Then you fall asleep.

Since you're no longer blinking, that cleanup system pauses. The mucus, oils, and other debris continue to collect, and as some of the water evaporates overnight, the mixture becomes thicker and forms the familiar crust you wipe away in the morning.

A small amount is completely normal and usually means your eyes' natural cleaning and lubrication system is doing exactly what it should.