Blood oxygen sensing is disabled in the US (due to the patent lawsuit with Massimo), but it still estimates VO2Max and sleep stages. VO2Max is based on GPS and

Blood oxygen sensing is disabled in the US (due to the patent lawsuit with Massimo), but it still estimates VO2Max and sleep stages. VO2Max is based on GPS and heart rate sensor measurements, processed by a deep learning algorithm that uses physiological ODEs (ordinary differential equations). Very cool algorithm. Apple Watch sleep stages (and sleep apnea detection) is based only on 3D accelerometer data.