Implementations disclosed herein provide a hydration monitoring technology. In one implementation, a hydration monitoring system measures whole body hydration levels by analysis of changes in vascular volume caused by pulsatile pressure waves and in tissue volume in response to the pulsatile pressure. The hydration monitoring system includes a hydration monitoring device, which uses a light-based measurement technique to measure hydration levels and heart rate during different activities and at rest. In one implementation, a light source operatively connected to a light sensor, transmits light, reflectively or transmissively, through tissue. The light sensor detects absorption of the light. Based on wavelength measurements of the detected light, the hydration monitoring device produces a PPG waveform representing characteristic effects of hydration. Based on analysis of the PPG waveform, the hydration monitoring device determines a hydration metric representative of hydration levels in the body.
