The Sleeping Worlds (夢見る世界, Yumemiru Sekai?) are the main setting of Kingdom Hearts 3D: Dream Drop Distance. They are worlds that fell into darkness but were not fully revived following the defeat of Ansem, Seeker of Darkness for unknown reasons. As this created barriers not even the Heartless could penetrate, a new form of darkness emerged in the Sleeping Worlds, known as the Dream Eaters.
The nature of the Sleeping Worlds is complex as time is not of any concern as the worlds are a "dream" of sorts that depict events that occurred in the worlds before they were consumed. In fact, in the case of Mickey Mouse and Jiminy Cricket, those who were in a world but not present when it was consumed in darkness are recreated within the Sleeping World as how it remembers them.
Kingdom Hearts 3D: Dream Drop Distance
As part of their Mark of Mastery exam, Yen Sid sends Sora and Riku to the Sleeping Worlds to unlock the seven Keyholes of Sleep, and return the worlds to the Realm of Light in order to obtain a power to defeat Xehanort. However, Xehanort foresaw such an event and arranged for his younger self to appear in the sleeping worlds. Eventually, Sora and Riku manage to seal the seven Keyholes in the Sleeping Worlds, but thanks to a set-up by the real Organization XIII, Sora ends up trapped in the World That Never Was and his heart is damaged during a fight with Xemnas. With help from Mickey, Donald, Goofy, and Lea, Riku manages to save Sora from becoming a new vessel for a fragment of Master Xehanort's heart, and they return safely to the Realm of Light.