The purpose of this study was to evaluate the effectiveness of mandibular advancement devices ( MADs ) for treatment of obstructive sleep apnea syndrome ( OSAS ) compared with the results obtained with a placebo device in accordance with the following indicators : apnea hypopnea index ( AHI ) per hour of sleep , apnea index ( AI ) per hour of sleep , mean oxyhemoglobin saturation , sleep efficiency , and percentage of rapid eye movement ( REM ) sleep .