A sleep monitoring device (100) is disclosed for measuring a biophysical variable (131) of a living-being (150), comprising at least two pressure sensors (101-103) spatially arranged in a predefined planar geometry. The living-being rests on the supporting layer (199) contacted by the sensors that generate sensor signals in response to pressure caused by the living being via the supporting layer. A processing unit (110) determines the position of the living-being on the supporting layer based on differences between the sensor signals; a magnitude attenuation factor is determined based on the position of the living-being and a position of a sensor; the biophysical variable is determined from a sensor signal generated by that sensor, including a magnitude correction based on the magnitude attenuation factor. The effect of the invention is that only few sensors are needed to measure a biophysical variable accurately, irrespective of the living-being's position on the supporting layer.