  We consider an atom in interaction with a massless scalar quantum field. We
discuss the structure of the rate of variation of the atomic energy for an
arbitrary stationary motion of the atom through the quantum vacuum. Our main
intention is to identify and to analyze quantitatively the distinct
contributions of vacuum fluctuations and radiation reaction to the spontaneous
excitation of a uniformly accelerated atom in its ground state. This gives an
understanding of the role of the different physical processes underlying the
Unruh effect. The atom's evolution into equilibrium and the Einstein
coefficients for spontaneous excitation and spontaneous emission are
calculated.
