Realm of Sleep

The Realm of Sleep makes an appearance in Kingdom Hearts 3D: Dream Drop Distance. The Realm of Sleep is a realm, which is forever sleeping. The Realm of Sleep is filled with eight sleeping worlds. The eight sleeping worlds are Traverse Town, La Cité des Cloches, Prankster's Paradise, The Grid, Symphony of Sorcery, Country of the Musketeers and The World That Never Was.

Separation
There are four seperate groups of the sleeping worlds. Firstly, Traverse Town is by itself. La Cité des Cloches, Prankster's Paradise and The Grid are next. Country of the Musketeers and Symphony of Sorcery are right after. Then, Lastly, The World That Never Was. These worlds are forever sleeping in the Real of Sleep.

Mark of Mastery Exams
For the Mark of Mastery Exams, Sora and Riku were sent to the Realm of Sleep in search for the seven Sleeping Keyholes. They both arrived in two seperate Traverse Towns. Sora is trapped in one Traverse Town and Riku is trapped in the other. There are also duplicates of the other sleeping worlds in the Realm of Sleep, which causes Sora and Riku to be appart. Unlocking the seven Sleeping Keyholes will free Sora and Riku from their wakelessness in the Realm of Sleep.

Enemies
In the Realm of Sleep, where the Heartless can not enter, there is still a manner of darkness. They're called Dream Eaters. Dream Eaters are the source of darkness that lurk in the sleeping worlds. There are two types of Dream Eaters, Spirits (which are a source of light) and Nightmares (the source of darkness).