An upper tray adapted to receive a wearer's maxillary teeth. A lower tray adapted to receive the wearer's mandibular teeth. A first upper ridge is carried on a maxillary occlusal surface of the upper tray. A second upper ridge is carried on the maxillary occlusal surface of the upper tray posterior to and spaced from the first upper ridge. A lower ridge is carried on a mandibular occlusal surface of the lower tray received between the first upper ridge and the second upper ridge. The lower tray is movable to a forward position where the lower ridge abuts the first upper ridge to limit forward movement of the lower tray in relation to the upper tray, and the lower tray is movable to a rearward position where the lower ridge abuts the second upper ridge to limit rearward movement of the lower tray in relation to the upper tray.
