A mattress includes a first, phase changeable component and a thermal management system adapted to control the temperature of the phase changeable component in order to change the firmness of the mattress. In one embodiment the phase changeable component includes two dimensional lattices separated from each other and a spring element extending between the lattices. Some embodiments of the mattress include an optional single phase component. The firmness can be made spatially and/or temporally uniform or nonuniform, and can be controlled based on one or more sensed parameters.
