It didn't stop demons from coming, but in a room full of potentially frightened or worried children, perhaps a little homesick, if they felt at ease when asleep, nothing did come... and that was because they felt safe in their beds. Make a Circle into a prison and it's ripe for demons.

Hard to say whether it could work on not-actually-an-archdemon nightmares.

"Sooner than later," Sabriel encourages, though her smile doesn't quite return as she lowers the tea down to her lap. Wait a minute. "Wait. Those rooms with broken walls..."