When the Doctor described this monster as ‘sentient sleep dust’ I honestly thought he was joking! The walking CG zombies were suitably scary and the whole transmission thing was cool, but the whole concept was ridiculous even by Who standards.
Sandmen was the given name of a dangerous side effect of the Morpheus project which helped people live without sleep. Collective sleep dust accumulated and gained sentience – able to appear as humanoid forms or even completely human. More dangerously they were able to transmit the process that created them as information.