A support device for supporting the body of a person, the support device comprising: an inflatable layer formed of a plurality of inflatable cells communicating with inflation and deflation means; a sensor pad arranged below the inflatable layer, the sensor pad comprising at least three sensing elements configured to sense the load applied to each sensing element, each sensing element being a capacitive sensing element, the three sensing elements being separated from one another in a direction along the width of the support device, and the three sensing elements comprising: a left sensing element arranged below a left hand portion of the inflatable layer, a middle sensing element arranged below a central portion of the inflatable layer, and a right sensing element arranged below a right hand portion of the inflatable layer; and electric circuitry connected to each sensing element of the sensor pad, the electric circuitry being configured to: control the inflation and deflation means for filling or emptying, respectively, the inflatable cells of the inflatable layer in such a way that the internal air pressure of the inflatable cells is a function of the load of the body of a person resting on the support device as sensed by each sensing element; determine the position on the support device of a person resting on the support device based on the load sensed by each sensing element; combine the load sensed by each sensing element and determine the total load of a person resting on the support device based on the combined load measurements; compare the load sensed by the middle sensing element to the determined total load of a person resting on the support device; and determine that the person resting on the support device is in an exit position on the support device if the load sensed by the middle sensing element is less than a predetermined fraction of the determined total load. an inflatable layer formed of a plurality of inflatable cells communicating with inflation and deflation means; a sensor pad arranged below the inflatable layer, the sensor pad comprising at least three sensing elements configured to sense the load applied to each sensing element, each sensing element being a capacitive sensing element, the three sensing elements being separated from one another in a direction along the width of the support device, and the three sensing elements comprising: a left sensing element arranged below a left hand portion of the inflatable layer, a middle sensing element arranged below a central portion of the inflatable layer, and a right sensing element arranged below a right hand portion of the inflatable layer; and a left sensing element arranged below a left hand portion of the inflatable layer, a middle sensing element arranged below a central portion of the inflatable layer, and a right sensing element arranged below a right hand portion of the inflatable layer; and electric circuitry connected to each sensing element of the sensor pad, the electric circuitry being configured to: control the inflation and deflation means for filling or emptying, respectively, the inflatable cells of the inflatable layer in such a way that the internal air pressure of the inflatable cells is a function of the load of the body of a person resting on the support device as sensed by each sensing element; determine the position on the support device of a person resting on the support device based on the load sensed by each sensing element; combine the load sensed by each sensing element and determine the total load of a person resting on the support device based on the combined load measurements; compare the load sensed by the middle sensing element to the determined total load of a person resting on the support device; and determine that the person resting on the support device is in an exit position on the support device if the load sensed by the middle sensing element is less than a predetermined fraction of the determined total load. control the inflation and deflation means for filling or emptying, respectively, the inflatable cells of the inflatable layer in such a way that the internal air pressure of the inflatable cells is a function of the load of the body of a person resting on the support device as sensed by each sensing element; determine the position on the support device of a person resting on the support device based on the load sensed by each sensing element; combine the load sensed by each sensing element and determine the total load of a person resting on the support device based on the combined load measurements; compare the load sensed by the middle sensing element to the determined total load of a person resting on the support device; and determine that the person resting on the support device is in an exit position on the support device if the load sensed by the middle sensing element is less than a predetermined fraction of the determined total load.