An oral treatment device comprising: an intraoral mouthpiece having a dental arch midline plane and comprising: a lamp support structure comprising: a curved support plate; a first relief element formed in the curved support plate on a first side of the dental arch midline plane that increases flexibility of a first end portion of the curved support plate relative to a central portion of the curved support plate; and a second relief element formed in the curved support plate on a second side of the dental arch midline plane that increases flexibility of a second end portion of the curved support plate relative to the central portion of the curved support plate; and a lamp mounted to the lamp support structure and configured to emit electromagnetic radiation onto oral surfaces when the intraoral mouthpiece is positioned within a mouth of a user and activated; characterized in that the curved support plate is formed of a first material having a first hardness and the first and second relief elements (127, 128) are sealed with a second material having a second hardness which is less than the first hardness. an intraoral mouthpiece having a dental arch midline plane and comprising: a lamp support structure comprising: a curved support plate; a first relief element formed in the curved support plate on a first side of the dental arch midline plane that increases flexibility of a first end portion of the curved support plate relative to a central portion of the curved support plate; and a second relief element formed in the curved support plate on a second side of the dental arch midline plane that increases flexibility of a second end portion of the curved support plate relative to the central portion of the curved support plate; and a lamp mounted to the lamp support structure and configured to emit electromagnetic radiation onto oral surfaces when the intraoral mouthpiece is positioned within a mouth of a user and activated; a lamp support structure comprising: a curved support plate; a first relief element formed in the curved support plate on a first side of the dental arch midline plane that increases flexibility of a first end portion of the curved support plate relative to a central portion of the curved support plate; and a second relief element formed in the curved support plate on a second side of the dental arch midline plane that increases flexibility of a second end portion of the curved support plate relative to the central portion of the curved support plate; and a curved support plate; a first relief element formed in the curved support plate on a first side of the dental arch midline plane that increases flexibility of a first end portion of the curved support plate relative to a central portion of the curved support plate; and a second relief element formed in the curved support plate on a second side of the dental arch midline plane that increases flexibility of a second end portion of the curved support plate relative to the central portion of the curved support plate; and a lamp mounted to the lamp support structure and configured to emit electromagnetic radiation onto oral surfaces when the intraoral mouthpiece is positioned within a mouth of a user and activated; characterized in that the curved support plate is formed of a first material having a first hardness and the first and second relief elements (127, 128) are sealed with a second material having a second hardness which is less than the first hardness.