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From Discworld & Pratchett Wiki

On Roundworld, a being that gently fills children's eyes with sleeping sand from his magic sack in order for them to drift off to the land of Nod with beautiful dreams.

On Discworld, however, the Sandman doesn't take the sand out of the sack, thereby rendering it a cosh with which he belabours his victims. Presumably the outcome is more unconsciousness than sleep, but the effect is similar from a distance...