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Often, after waking up from a night's sleep hard, flaky, pale yellow crystals are deposited around the eyelids (which easily wash off with water).

What are these? Why are they yellow? What are they made of? Is their amount proportional to the length of sleep?

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This is quite common. It is most likely dried rheum (also know by slang terms such as "sleep", "eye boogers", "eye crusties", etc.).

Rheum is the result of dried mucus, tears (and the salt and minerals therein), as well as dead blood and skin cells.

Essentially it is all the stuff that’s normally in your eye that eventually dries up making the contents more condensed giving it a light yellow color. When all that stuff is in your tears it is spread out enough that your tears are still mostly clear.

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