The Akhmediev breather (AB) and its M-soliton generalization, hereafter called AB M , are exact solutions of the focusing NLS equation periodic in space and exponentially localized in time over the constant unstable background; they describe the appearance of M unstable nonlinear modes and their interaction, and they are expected to play a relevant role in the theory of periodic anomalous (rogue) waves (AWs) in nature. It is therefore important to establish the stability properties of these solutions under perturbations, to understand if they appear in nature, and in which form. It is rather well established that they are unstable with respect to small perturbations of the NLS equation. Concerning perturbations of these solutions within the NLS dynamics, there is the following common believe in the literature. Let the NLS background be unstable with respect to the first N modes; then i) if the M unstable modes of the AB M solution are strictly contained in this set (M < N ), then the AB M is unstable; ii) if they coincide with this set (M = N ), the so-called "saturation of the instability", then the AB M solution is neutrally stable. In this paper we argue instead that the AB M solution is always unstable, even in the saturation case M = N , and we prove it in the simplest case M = N = 1. We first prove the linear instability, constructing