  Photonic crystals are periodic systems that consist of dielectrics with
different refractive indices. They are designed to act on photons in contrast
to semiconductor crystals whose periodicity affects the motion of electrons.
Here we consider the interaction of an atomic electron with its own radiation
field in the case when the atom is placed in air voids of a photonic crystal
and is not in mechanical contact with the vibrational degrees of freedom of the
dielectric host. A strong modification of this interaction from that in free
space is shown to change the rest mass of the electron, and this has a
significant effect on the shift of the atomic energy levels. This shift is
investigated by using the example of atomic hydrogen in a high-index-contrast
photonic crystal. The found effect may be of interest both from fundamental and
practical points of view.
