The tounge muscle is always tensed one way or another. Whilst it wouldn't slip all the way down your gullet to be digested in your stomach it when fully relaxed, which only happens when unconscious (not asleep) there is a risk that it can slip back and block your throat passage. This phenomenon is known as "swallowing your tongue" on the multiple basic first aid courses I have attended as a thing you check for when rendering first aid to someone that's unconscious: check for blocked air passages and remove visible blockages including the tongue if it has slipped back.