Waking up with crusty yellow gunk in the corners of your eyes is something we’ve all experienced — but most people have no idea what it actually is.

That sticky substance is medically known as “rheum,” and it’s a mix of mucus, dead skin cells, oils, and tiny particles like dust. Your eyes produce it constantly, but while you’re awake, blinking washes it away. At night, with your eyes closed, it collects — forming that familiar yellowish crust by morning.

While generally harmless, excessive or discolored eye discharge can sometimes signal an issue like conjunctivitis (pink eye), allergies, or a blocked tear duct. If you notice greenish color, pain, or blurry vision, it’s worth checking in with a doctor.

For most of us, though, it’s just your eyes doing their nightly cleaning job.
Gross? Maybe. But it’s a sign your body is working exactly as it should.