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 both blood and 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 Clara Oswald dubbed "Sandmen". (TV: Sleep No More)
The Eighth Doctor thought that the diamond-like object (actually the Bruce Master's trap) he found in the corner of the Eye of Harmony was irritating the Eye like "sleep" did to human eyes. (PROSE: The Eight Doctors)