A method for non-invasive measurements of biomarkers, comprising: providing a case, wherein the case is a wrist-watch back case to replace a removable back case of a timekeeping device or to be attached thereto, the wrist-watch back case comprising: a casing comprising a bottom wall and a side wall thereby defining an inner empty space, at least five openings (103; 104) at the bottom wall, an optoelectronic circuit board to be fitted in the inner space of the casing the board comprising, at least two paired radiation sources (303A; 303B) to impinge radiation on a user's skin when the wrist-watch is worn by the user, each source having a different emission wave length to obtain a different degree of skin penetration, at least one radiation detector to detect a reflected radiation signal exiting from the skin and to transform the reflected radiation signal into a processable signal, wherein the radiation sources and the radiation detector are arranged in correspondence to the at least five openings, and the wrist-watch back case also comprises radiation guiding elements to provide a coupling of the reflected radiation, wherein the guiding elements are arranged between the radiation detector and the user's skin; illuminating a skin with the at least two radiation sources; capturing the radiation exiting from the skin; generating an absorption signal; extracting the epochs from the PPG signal; identifying the pulses of each epoch on the PPG signal; filtering the PPG signal by rejecting non-acceptable pulses; repeating the filtering step for all pulses until each pulse is within a predetermined reliability range; calculating an artefact-free derived PPG signal (PPGd), and extracting features from the PPGd signal, wherein extracting features comprises: providing a PPGd signal having a plurality of epochs comprising pulses; separating the pulses of each epoch of the PPGd signal, further comprising for each pulse: extracting a falling edge (ePW(i)) of the pulse; discarding beginning and end tails of each ePW(i); calculating a first derivative of each ePW(i); finding zero crossings (zCx) of each ePW; determining a location of the zero crossings (zCx); retrieving all dicrotic notch/ incisura positions; and repeating the steps for all pulses of the PPGd signal. providing a case, wherein the case is a wrist-watch back case to replace a removable back case of a timekeeping device or to be attached thereto, the wrist-watch back case comprising: a casing comprising a bottom wall and a side wall thereby defining an inner empty space, at least five openings (103; 104) at the bottom wall, an optoelectronic circuit board to be fitted in the inner space of the casing the board comprising, at least two paired radiation sources (303A; 303B) to impinge radiation on a user's skin when the wrist-watch is worn by the user, each source having a different emission wave length to obtain a different degree of skin penetration, at least one radiation detector to detect a reflected radiation signal exiting from the skin and to transform the reflected radiation signal into a processable signal, wherein the radiation sources and the radiation detector are arranged in correspondence to the at least five openings, and the wrist-watch back case also comprises radiation guiding elements to provide a coupling of the reflected radiation, wherein the guiding elements are arranged between the radiation detector and the user's skin; illuminating a skin with the at least two radiation sources; capturing the radiation exiting from the skin; generating an absorption signal; extracting the epochs from the PPG signal; identifying the pulses of each epoch on the PPG signal; filtering the PPG signal by rejecting non-acceptable pulses; repeating the filtering step for all pulses until each pulse is within a predetermined reliability range; calculating an artefact-free derived PPG signal (PPGd), and extracting features from the PPGd signal, wherein extracting features comprises: providing a PPGd signal having a plurality of epochs comprising pulses; separating the pulses of each epoch of the PPGd signal, providing a PPGd signal having a plurality of epochs comprising pulses; separating the pulses of each epoch of the PPGd signal, extracting a falling edge (ePW(i)) of the pulse; discarding beginning and end tails of each ePW(i); calculating a first derivative of each ePW(i); finding zero crossings (zCx) of each ePW; determining a location of the zero crossings (zCx); retrieving all dicrotic notch/ incisura positions; and repeating the steps for all pulses of the PPGd signal.