  We consider a hydrogen atom confined in a thick brane embedded in a
higher-dimensional space. Due to effects of the extra dimensions, the
gravitational potential is amplified in distances smaller than the size of the
supplementary space, in comparison with the Newtonian potential. Studying the
influence of the gravitational interaction modified by the extra dimensions on
the energy levels of the hydrogen atom, we find independent constraints for the
higher-dimensional Planck mass in terms of the thickness of the brane by using
accurate measurements of atomic transition frequencies. The constraints are
very stringent for narrow branes.
