A mattress core has front and rear ends that are spaced apart in a longitudinal direction and further has opposite sides that are spaced apart in a lateral direction. The core includes a firmer lower core section having a top surface that follows an undulating pattern. The undulating pattern is non-uniform and non-symmetric in the longitudinal direction, is uniform in the lateral direction, and includes at least one hill, at least one valley and at least one plateau. The core further includes a softer upper core section that overlies the lower core section. The upper core section has a bottom surface that follows the undulating pattern and contacts the lower core section's top surface to provide a common mating interface that follows the undulating pattern. The interface includes at least one hill, at least one valley and at least one plateau.