  We study the spontaneous de-excitation and excitation of accelerated atoms on
arbitrary stationary trajectories (``generalized Unruh effect''). We consider
the effects of vacuum fluctuations and radiation reaction separately. We show
that radiation reaction is generally not altered by stationary acceleration,
whereas the contribution of vacuum fluctuations differs for all stationary
accelerated trajectories from its inertial value. Spontaneous excitation from
the ground state occurs for all { accelerated stationary} trajectories and is
therefore the ``normal case''. We furthermore show that the radiative energy
shift (``Lamb shift'') of a two-level atom is modified by acceleration for all
stationary trajectories. Again only vacuum fluctuations give rise to the shift.
Our results are illustrated for the special case of an atom in circular motion,
which may be experimentally relevant.
