Methods and systems for diagnosing or assessing rapid eye movement sleep behavior disorder (RBD). Muscle tone or activity variance during rapid eye movement (REM) and non-rapid eye movement (NREM) sleep intervals of a polysomnogram are compared. A threshold based on the NREM data is used to identify a subject-specific threshold for abnormality in the REM variance. A metric that includes the percentage of REM variance exceeding the threshold relates to RBD.