A system (100) for motion artifact reduction in a photoplethysmogram, PPG, signal representing heart activity of a subject, comprises: a processing unit (120), said processing unit (120) being configured to: receive (302) a PPG signal carrying information of heart activity of the subject; receive (304) a motion reference signal carrying information of a motion of the subject; form (306) a PPG time-frequency representation, based on the PPG signal, and a motion reference time-frequency representation, based on the motion reference signal; and subtract (308) a weighted representation of the motion reference time-frequency representation from the PPG time-frequency representation for reducing a motion artifact component in the PPG time-frequency representation and form a cleaned time-varying frequency spectrum of the PPG signal.