An on-body sensor unit (12) comprises a PPG sensor and an ECG sensor, each having sensor parts exposed at a skin-contacting surface (16) of the unit. The PPG includes an optical detector (22) and at least one light source (20a, 20b). Disposed between the at least one light source and the optical detector is a light-intercepting surface element (28), adapted to be absorptive of light of at least a majority of a spectral composition which is generated by the at least one PPG light source (20a, 20b). Located within, or forming, this surface element is an electrode (32a, 32b) comprised by the ECG sensor, said electrode having at least an exposed skin-interfacing surface which is absorptive of a majority of the spectral composition of said light generated by the at least one light source.
