I think Watanuki's "real" world might be the spirit realm, where he's met with Clone!Sakura's spirit, sleeping not!Syaoran, and Haruka Doumeki. He's often shown in pajamas there. When he sleeps, he goes to the life we've seen, with his friends and Yuuko's shop.

I believe that world of XXXHolic is a genuine reality. But Watanuki is like... a spirit or soul who is dreaming that he's alive. Once he discovers whatever the whole truth behind the situation is, his ability to stay in the XXXHolic reality will rest upon the power of his friends' belief in his existence. And, uh, Doumeki's egg trick.