Dream Drop Distance What exactly is a sleeping world?

Alright, so granted, I'm not that far into the game. I've got Traverse Town and Cite des Cloches beat and Sora's half of the Grid. But I still can't understand what exactly is a sleeping world. Traverse Town made me think that a world that's stuck in dreamland is split into two versions that have certain overlapping segments, but it seems like Grid and Cite have two versions that aren't overlapping much. Sora and Riku are going through two different versions of each world, but the inhabitants -save for Traverse Town- don't seem to realize that the world isn't awake. It's not stuck in some form of stasis as people are going on with their lives and things change and they learn new things. What makes it sleeping? Why are there exactly two versions? Why exactly can't Heartless and Nobodies find their way there if Nobodies come from nothingness and the Heartless taking those worlds over is why the worlds are asleep? How exactly are these "sleeping" worlds different from "waking" worlds?

 

Without spoiling too much, Traverse Town is a special case. The rest of the worlds aren't really separate versions per se, and most of Sora and Riku's respective escapades in each either don't overlap at all or complement each other.

 

All the "realm of sleep" is, is dreams. Worlds can dream and people can dream. When a world falls to darkness/goes to the Realm of Darkness, they also fall asleep and they dream. The worlds we go to in Dream Drop Distance are dreams of worlds that were freed from darkness, but were not freed from their sleep. Yen Sid sent Sora and Riku back to Destiny Islands before it fell to darkness, so that they could sort of "ride" Destiny Islands into darkness, therefore ride it into sleep. Once they rode it into sleep and fell asleep, they unknowingly opened the first Sleeping Keyhole and were transported to the next sleeping world/the first sleeping world from the present-day that they need to save.

There are two versions of each world in DDD because Riku isn't within the dreams of worlds, he's within Sora's dreams. When they entered the realm of sleep, Riku unknowingly dove into Sora's dreams and became a Dream Eater to protect Sora from Nightmares. So, Sora is traveling through the dreams of worlds that were freed from darkness, but were not freed from their sleep, while Riku is traveling through Sora's dreams.