A photoplethysmography (PPG) circuit obtains PPG signals at a plurality of wavelengths of light reflected from tissue of a user. A processing device generates parameters using the PPG signals to determine a glucose level in blood flow of the user. The parameters include one or more ratio values obtained using the plurality of PPG signals； a phase delay between the plurality of PPG signals； a correlation of phase shape between the plurality of PPG signals or a periodicity of one or more of the plurality of PPG signals.
