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Rheum, also called sleep dust, was a crusty, sand-like substance discharged from humans during sleep and deposited in the corners of their eyes. It was made of blood cells, skin cells and mucus.
In the 38th century, Professor Gagan Rassmussen's experiments with improving Morpheus pods resulted in the creation of life-forms made of sleep dust, which the Twelfth Doctor dubbed "Sandmen". (TV: Sleep No More)