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Rheum (//; from Greek: ῥεῦμα rheuma 'a flowing, rheum') is a thin mucus naturally discharged from the eyes, nose, or mouth during sleep (cf. mucopurulent discharge). Rheum dries and gathers as a crust in the corners of the eyes or the 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 the conjunctiva), nasal mucus, blood cells, skin cells, or dust. Rheum from the eyes is particularly common. Dried rheum is commonly called sleep, sleepy-seeds, sleepy buds, sleepy sand, eye goop, sleep dust,  or sleepy dirt. 
When the individual is awake, blinking of the eyelid causes rheum 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 rheum accumulating in corners of the eye, most notably in children.
A number of conditions can increase the production of rheum in the eye. In the case of allergic conjunctivitis, the buildup of rheum can be considerable, preventing the sufferer from 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. In infants, the tear ducts (that drain the tears) occasionally fail to open, resulting in the overflow of tears onto the cheeks (epiphora) and rheum deposition on the surrounding skin.
- "Rheum (discharge)". Memidex Dictionary/Thesaurus. 2009. Retrieved 2009-04-22.
- Amodio, Aimee."Where Do Eye Boogers Come From?", Families.com blog
- Hiskey, Daven. "What the 'Sleep' In Your Eyes Is", Today I Found Out, 23 February 2011.
- Shorter Oxford English Dictionary. Oxford University Press. 2007. ISBN 978-0-19-920687-2.
- Eric Partridge, Paul Beale, ed., A Dictionary of Slang and Unconventional English, 8th edition, 1984, s.v.
- Nierenberg, Cari (14 September 2011). "Are yours crusty or wet? The truth behind eye boogers (ew)". NBC News.
- https://m.poemhunter.com/poem/the-eye-crud-the-sleepy-dirt/. Missing or empty
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