A bed was equipped with four force transducers so that the location of the Center Of Mass (COM) can be computed, when the bed contains a person. The computation of the COM and its alterations in combination with the sum of all measured forces allows to compute the person's position in bed, an activity level, the resulting body weight and the corresponding weight change rate over time (overnight and long-term) as well as simply to determine whether the person is in the bed or not. The results of several overnight measurement experiments show, that the person's position in the bed (central, left, right) can be correctly detected, the proposed activity level is a promising indicator for the sleep-activity and furthermore the initial body weight as well as the nightly weight change rate can be determined.