The time-dependent bending recovery of human hair fibers was investigated for a variety of relative humidities and aging times. The data were analyzed on the basis of a viscoelastic filament/matrix model and the Denby-equation, containing the parameter K as the ratio of the elastic bending rigidities of the matrix and the filaments and the Kohlrausch-Williams-Watts (KWW) function as relaxation function. The first stage of the analysis ascertained that the recovery curves shift with aging time on the time scale with the expected aging rate of mu approximate to 1. The second stage showed that the shape factor of the KWW function exhibits a mean value across the aging and humidity range of m = 0.28, which is close to the "universal" value of 1/3. On this basis, it was found that virtually no change Occurs for the modulus ratio for low water contents up to about 10%, being constant at K-0 = 6.1, while linearly decreasing beyond this threshold. The reduced, characteristic relaxation time drops on the log-time scale from log tau(r)(0) = 0.47 for the dry fiber linearly with water content, covering about two thirds of a decade for 0-20%, water content. With the pronounced humidity dependence of the parameters, hair shows what is termed hydro-rheologically complex (HRC) in analogy to thermo-rheologically complex behavior. Using the HRC approach, the dynamical mechanical performance of hair (1 Hz) was calculated for a range of water contents and aging times and found to be in good general agreement with experimental data. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113:3336-3344,2009