  We obtain a general result for the Lamb shift of excited states of
multi-level atoms in inhomogeneous electromagnetic structures and apply it to
study atomic hydrogen in inverse-opal photonic crystals. We find that the
photonic-crystal environment can lead to very large values of the Lamb shift,
as compared to the case of vacuum. We also predict that the position-dependent
Lamb shift should extend from a single level to a mini-band for an assemble of
atoms with random distribution in space, similar to the velocity-dependent
Doppler effect in atomic/molecular gases.
