They go by many names — crusties, eye boogers, sleep dust, goop, or sometimes simply “sleep.” But whatever you call it, you’ve experienced this: you wake up in the morning and, in the corner of your eyes, there are shards of a hard, yellowish-white crust hanging out. Why does this happen?
The short version: When we sleep, we don’t blink.
Eye crusties (or whatever you call this stuff) are made up of something called rheum, pronounced like the word “room,” which Wikipedia describes as a “thin mucus naturally discharged from the eyes.” Rheum protects our eyes from dust and whatever other bad stuff would otherwise irritate our eyes — it’s a barrier which catches the bad stuff before it causes us any problems. Of course, we don’t want specks of dust or whatever floating around our eyes, so we have to get rid of the rheum pretty often. Blinking takes care of this. Blinking moves tears from the outside of the eyes inward, toward our tear ducts. And when that happens, the tears wash the rheum away ever so subtly; unless you’re thinking about it, you probably don’t even notice it happening.
We make rheum awake or asleep — if there’s something bothering our eyes, the mucus layer is there to help, whether it’s 3 AM or 3 PM. As Dr. Sherleen Chen, a professor of ophthalmology at the Massachusetts Eye and Ear Infirmary, explained to NBC News, when we’re asleep, the “dirt and debris within the eye isn’t continually washed over by tears, which would help to dilute them. So at night, dryness causes the stuff in tears to precipitate out,” and it has to go somewhere — “the crud collects toward the inside corner of the eye, where tears usually end up.” And as a result, as Mental Floss explains, the rheum “dries out and hardens, leaving you looking like your face planted in a sandcastle sometime during the night.”
By and large, eye crusties are basically harmless — it’s safe to gently wipe away when you wake up. Unless there’s an excessive amount of crust, all it means is that your eyes were fighting off dust or something similar while you were dreaming.
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Related: Eye crust remover for pets — don’t use it on yourself.