Sleep dust

Sleep dust was a crusty, sand-like substance resembling dust, discharged from humans during sleep and deposited in the corners of their eyes. It was made of blood and skin [cells together with 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)