  We present an ab-initio study of the electronic response function of sodium
in its 5 known metallic phases from 0 to 180 GPa at room temperature. The
considered formalism is based on a interpolation scheme within time-dependent
density functional theory that uses maximally localized Wannier functions,
providing an accurate sampling of the reciprocal space. Besides showing an
excellent agreement with inelastic X-ray scattering experiments, our
calculations reveal that the drastic decrease of the optical reflectivity
recently measured in the high pressure phases oP8 and tI19 is associated to a
new low-energy plasmon arising from collective interband excitations.
Additionally, our calculations predict the existence of an anisotropic
interband plasmon in the stability pressure range of fcc Na (65 to 105 GPa).
