A measuring device for detecting positional changes of persons in a bed, having a bridge (10), which is arranged in a middle region of the lying surface transversely to the bed and is supported there by at least two supporting regions spaced apart from each other. The bridge (10) supports a mattress lying on the lying surface (1). The measuring device includes a sensor (13, 13a) at at least one of the supporting regions. Through use of the measured values of the sensors (13, 13a) a controller (21) detects the center of gravity position of the weight distribution loading the bridge (10) and activates an alarm if the movement activity of a person lying on the bed is insufficient.
