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.
"Look, lad! You see the streak of glowing green off the larboard bow? Them's the bright waters!"
While the lad peered intently at the eerie glow, the old salt continued. "Entire ships have been lost to the Bright, lad, for once you let it surround you, you'll never touch land again!"