We study the Hamiltonian of a two-level system interacting with a one-mode radiation field by means of the Wigner method and without using the rotatingwave approximation. We show that a phenomenon of collapses and revival, reminiscent of that exhibited by the Jaynes-Cummings model, takes place in the high-coupling limit. This process appears as irreversible or virtually reversible, according to whether the semiclassical regime is chaotic or not. Thus, we find a new mechanism for dissipation in the quantum domain.