We make high-resolution simulations of poorly understood decays of nonlinear atmospheric acoustic–gravity waves (AGWs) after deactivations of the wave forcing. The standard deviations of AGW perturbations, after fast dispersions of traveling modes, experience slower exponential decreases. AGW decay times are estimated for the first time and are 20–100 h in the stratosphere and mesosphere. This requires slow, quasi-standing and secondary modes in parameterizations of AGW impacts to be considered.