The sticky, crusty material that collects at the corner of the eyes, commonly called “sleep crust” or “eye gunk,” is scientifically known as rheum. This discharge is a byproduct of the eye’s continuous self-cleaning process. Rheum functions as a mechanism to remove debris and waste that accumulates on the eye’s surface throughout the day. It is generally harmless when it appears as a small, dry crust upon waking.
What Rheum Is Made Of
Rheum is a mixture of substances produced by glands and cells within the eye and eyelid. The material primarily consists of mucus (mucin), secreted by specialized cells in the conjunctiva, the membrane covering the eye and inner eyelid. This mucus traps foreign particles and cellular waste for removal.
Tears, which are largely water, are produced by the lacrimal glands. An oily substance called meibum comes from the meibomian glands along the edges of the eyelids. While meibum normally prevents tear evaporation, excess amounts are found in the rheum.
The discharge also includes exfoliated skin cells from the eyelids and eye surface. Dust, pollen, or micro-particles that entered the eye during the day are swept up and bound within the rheum.
How Rheum Accumulates Overnight
During waking hours, blinking acts like a windshield wiper, constantly spreading the tear film and flushing away debris. Blinking moves accumulated rheum toward the tear ducts, draining the material into the nasal cavity. This action prevents the discharge from hardening.
When a person sleeps, the eyes are closed, and blinking stops. Without this movement and drainage, the debris-laden mixture pools in the inner corner of the eye (the canthus). The water content of the pooled discharge then gradually evaporates overnight.
Evaporation leaves behind the solid components—mucus, oils, and cellular debris—which dry and solidify into the crusty residue seen in the morning. The amount of rheum that forms can be influenced by environmental factors, such as dry air or allergens.
When Eye Discharge Signals a Problem
While a small amount of clear or whitish, dry crust is normal, changes in the nature of eye discharge can indicate an underlying health issue. Discharge that becomes yellow, green, or gray often signals a bacterial infection, such as bacterial conjunctivitis (pink eye). The consistency is also an important clue; abnormal discharge may be sticky, thick, or glue-like, sometimes causing the eyelids to be sealed shut upon waking.
A significant increase in the volume of discharge that occurs throughout the day, rather than just in the morning, also warrants attention. This excessive discharge can be a symptom of conditions like allergies, which typically produce a watery discharge, or a blocked tear duct, which can cause a gooey, sticky residue.
Problematic discharge is frequently accompanied by other symptoms, including noticeable redness of the eye, pain, a gritty or burning sensation, or increased sensitivity to light. Seeking medical evaluation is advised if the discharge is excessive, pus-like, or if it presents alongside vision changes or eye pain.