The invention provides mouth guards sized and configured to prevent jaw clenching and teeth grinding and methods of using the mouth guards. One embodiment provides a one-piece mouth guard made of a semi-rigid, springy, flexible polymer that includes an arcuate tray portion having a front, a rear, a top side and a bottom surface, the arcuate tray portion sized and configured to at least partially cover a person's upper tooth set, and an elastic lower tooth set-engaging tab portion extending from the front of the arcuate tray portion, which tab portion, in its relaxed state, further extends downward below the bottom surface of the tray portion and partially rearward toward the rear of the tray portion so that, rearward of the point the tab portion extends from the tray portion, a space is present between the bottom surface of the tray portion and the tab below.
