A question about Kingdom Hearts?

In the games where you play as Roxas (Kingdom hearts 358/2 Days, and the beginning of Kingdom Hearts II), Sora is in a deep slumber that he can't awake from until Roxas's memories are returned to him. How did Sora fall into this sleep? I know DiZ (Ansem the Wise in disguise) and Namin� had something to do with it, but I don't think Kingdom Hearts 1 ever made it clear as to how it happened. I know it isn't important at all as of how it happened, I'm just curious. Maybe the explanation is in Re: Chain of Memories... If so, just let me know, and I might buy that game as well. If not, then tell me your oppinion on how it happened... or just tell me if you know exactly how!