  We study the time evolution of an atom suddenly coupled to a thermal
radiation field. As a simplified model of the atom-electromagnetic field system
we use a system composed by a harmonic oscillator linearly coupled to a scalar
field in the framework of the recently introduced dressed coordinates and
dressed states. We show that the time evolution of the thermal expectation
values for the occupation number operators depend exclusively on the
probabilities associated with the emission and absorption of field quanta. In
particular, the time evolution of the number operator associated with the atom
is given in terms of the probability of remaining in the first excited state
and the decay probabilities from this state by emission of field quanta of
frequencies $\omega_k$. Also, it is showed that independent of the initial
state of the atom, it thermalizes with the thermal radiation field in a time
scale of the order of the inverse coupling constant.
