A non-invasive method to determine a current functional blood oxygen saturation level: acquiring, at a sampling frequency, a photoplethysmography light response for each wavelength of a plurality of at least three wavelengths, resulting respectively in a first, second and third raw data series, applying a low -pass filter to each of the first, second and third raw data series, resulting respectively in a first, second and third DC data series representing an unmodulated amplitude response determining respectively in a first, second and third AC data series representing an unmodulated amplitude response, applying a. computation engine to the first, second and third AC and DC data series, and determining therefrom a user current blood oxygen saturation level.