If you maintain your mash temps overnight, there shouldn't be a problem with souring. Lacto doesn't become a problem until you're below saccharification temps.
Mr. Wizard in BYO addressed this very question: http://byo.com/european-dark-lager/item/931-is-there-any-reason-you-couldnt-mash-overnight-and-just-sparge-in-the-morning

