https://www.mdpi.com/1424-8220/21/10/3571#table_body_display_sensors-21-03571-t001 Comment of the MRIS: The paper uses the WHOOP 2.0, but the latest device is WHOOP 4.0 Discussion «This study evaluated agreement between PPG assessment of HR and HRV by a wearable biosensor (WHOOP 2.0) and gold-standard assessment via ECG.» Sleep Stage Identification «Cardiac activity was assessed via two ECG electrodes (left-positive and right-negative) …