"Reills. E'en the name makes me shudder. That there is unholy ground, cursed by most gods. Not even the demons and devils and the incarnations o' evil dare step foot there. Reills. You know me, I think money likes it in my pocket instead of wherever else it is. But, and I've heard rumors that say this, if theirs treasure on that ground, it ain't going in my pocket on pain o' death."
-Old Gerald, man in the pub.
When you sleep, your soul briefly returns to the place where it was created/goes to when it dies, and seeks nourishment. Everybody's soul is created with a little link (or you could say a greater entity calls it back) to that place so that it can return easily, but upon death, the soul returns via that and annihilates the link as if returns.
Also, as an added thingy, you could say the entity supplying the link, or residue of the links themselves, (whichever) is the source of all arcane power.