Rheum ( /ˈruːm/; from Greek: ῥεῦμα, rheuma, a flowing, rheum) is thin mucus naturally discharged as a watery substance from the eyes, nose or mouth during sleep (contrast mucopurulent discharge). Rheum dries and gathers as a crust in the corners of the eyes or mouth, on the eyelids, or under the nose. It is formed by a combination of mucus (in the case of the eyes, consisting of mucin discharged from the cornea or conjunctiva), nasal mucus, blood cells, skin cells, or dust. Rheum from the eyes is particularly common and is in common usage called eye gunk, sleepydust, sleep, sleepys, sleepers, eye goop, eye crud, eye crust, eye bogeys, eye boogers, and others.
In the waking individual, blinking of the eyelid causes gound to be washed away with tears via the nasolacrimal duct. The absence of this action during sleep, however, results in a small amount of dry gound accumulating in corners of the eye, most notably in children.
A number of conditions can cause an increase in the production of gound in the eye. In the case of allergic conjunctivitis, the buildup of rheum can be considerable, many times preventing the sufferer opening the eye upon waking without prior cleansing of the eye area. The presence of pus in an instance of heavy rheum buildup can indicate dry eye or conjunctivitis, among other infections.
Occasionally, in infants in their first year, the tear ducts fail to open. This causes epiphora, that is, with non-patent nasolacrimal ducts, tears have no egress from the body and rheum is released onto surrounding skin.