Implementations disclosed herein provide a hydration monitoring technology. In one implementation, a hydration monitoring device 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 device uses a light-based measurement technique to transmits light, reflectively or transmissively, through tissue. Based on wavelength measurements of the detected light, the hydration monitoring device produces a PPG waveform representing characteristic effects of hydration. The hydration monitoring device monitors operating condition signals associated with the hydration monitoring device. By monitoring operating condition signals, the hydration monitoring device determines whether the operating condition signals satisfy an analysis condition, and modifies the optical sensing operations. Based on the modified optical sensing operations, the PPG waveform is analyzed, and the hydration monitoring device determines a hydration metric representative of hydration levels in the body of the subject.
