We use spectroscopic ellipsometry to obtain the complex refractive index, ñ=n+ik, of human fingernails. By studying the change of ñ upon hydration and dehydration, we reveal three different time domains with typical time constants of 4, 150, and 3200 min. A simple model that takes into account the presence of one fast and one slow process is fully consistent with the observed hydration and dehydration dynamics. We attribute these processes to "free" water incorporated between the keratin filaments and water more tightly "bound" in keratin complexes, respectively. From our model we determine the hydration profiles of "free" and "bound" water during, both, hydration and dehydration.