The invention relates generally to bed monitoring device for monitoring bodily activity of a person and in particular vital signs such as respiration, cardiac activity and body's physical movement, including the presence, absence, step out or even falling out of a person using a sleeping environment such as a bed; mattress; top-mattress, bedding such as mattress covers, and pillow covers; a pad; a futon and the like. Based on the monitoring of bodily activity, the bed-monitoring device further allows determining sleep comfort and sleeping position of the user. It accordingly provides a valuable R&D tool to manufacturers of sleeping articles like beds, mattresses, top-mattresses, futons and the like; but also in counselling consumers when buying sleeping articles under professional guidance. The bed monitoring device of the present invention is characterized in comprising an optical fiber fixed to or mixed into a flat body positioned within said sleeping environment. This optical fiber enables detection of bodily activity by measuring a polarization variation of light propagating through said optical fiber. Further integration of this device with other monitoring systems such as home automation and alarm systems is also possible. Further integration of this device with other monitoring systems such as home automation, bed automation and control systems, and alarm systems is also possible.