plastic - metal hybrid materials are included in the construction of highway tractor sleeper sides and back panel assemblies . sleeper panels are traditionally constructed of two elements , the panel assemblies and the interior trim . a molded panel becomes both the interior of the structure and the decorative interior surface . this allows interior components to be directly mounted to the integral structural panels . the resulting sleeper will have stronger engagement to the mounted components such as cabinetry as well as part number reduction .