The present invention relates to a monitoring device and method. The proposed device comprises a light source (2) for emitting light pulses (3) into tissue (4) of a living being, a light sensor (5) for receiving light (6) from said tissue and generating a sensor signal (7), a filter unit (10) for filtering said sensor signal (7, 9), said filter unit comprising a switched in-phase low-pass filter (11) for generating an in-phase filter signal (12) and a switched out-of-phase low-pass filter (13) for generating an out-of-phase filter signal (14), a control unit (19) for controlling said light source (2) and said filter unit (10) such that the in- phase filter (11) is only switched on during a second time period (31) while the light source (2) is switched on and that the out-of-phase filter (13) is switched on during a first and third time period (30, 32) while the light source (2) is switched off, a subtraction unit (18) for subtracting the out-of-phase filter signal (14) from the in-phase filter signal (12).
