A non-invasive monitoring device for monitoring physiological condition of a subject may include at least one light source for emitting light at at least one predetermined narrow spectral wavelength band onto a living tissue of the subject and two or more light sensors. Each of the two or more light sensors may correspond to a specific substance in a living tissue of the subject and may be configured to measure a change in a concentration of the specific substances to which the sensor corresponds. The distance of each of the two or more sensors from the light source, may be determined so that intensity of light scattered by each specific substance, may have a maximal signal to noise ratio at the determined distance of the corresponding sensor.