The Discrete Nonlinear Schrödinger (DNLS) equation is a Hamiltonian model displaying frozen dynamics when breathers appear in the system. We study breather relaxation in the thermal region, T < +∞, in a unidirectional version of the DNLS equation, where the rest of the system does not feel the breather. Breather dynamics is governed by a time-dependent onedimensional Hamiltonian with two distinct time scales, and we show that the stability of the breather is related to the existence of an adiabatic invariant (AI). Approximate expressions for the AI are obtained both by implementing a canonical perturbation theory and a more phenomenological approach based on the estimate of the energy flux. The close correspondence with the original model allows excluding that breather destabilization is induced by the formation of localized bound states such as dimers. Finally, the AI dynamics reveals an unexpected similarity with Levy processes, which deserves further investigations.