Various perturbations (collisions, close encounters, YORP) destabilise the rotation of a small body, leaving it in a non-principal spin state. Then the body experiences alternating stresses generated by the inertial forces. The ensuing inelastic dissipation reduces the kinetic energy, without influencing the angular momentum. This yields nutation relaxation, i.e., evolution of the spin towards rotation about the maximal-inertia axis. Knowledge of the timescales needed to damp the nutation is crucial in studies of small bodies' dynamics. In the past, nutation relaxation has been described by an empirical quality factor introduced to parameterise the dissipation rate and to evade the discussion of the actual rheological parameters and their role in dissipation. This approach is unable to describe the dependence of the relaxation rate upon the nutation angle, because we do not know the quality factor's dependence on the frequency (which is a function of the nutation angle). This leaves open the question of relaxation timescale as a function of the nutation angle. This approach also renders a finite dissipation rate in the limit of a spherical shape. We study nutation damping, deriving our description directly from rheology. This gives us the dissipation rate and the nutation damping rate as functions of the shape and of the rheological parameters. Our development gives a zero damping rate in the spherical-shape limit. To simplify the developments, we consider a dynamically oblate rotator, and set the rheology Maxwell. Our method, however, is generic and applicable to any shape and any linear rheology. ; Comment: 19 pages, 4 figures