Spectroscopic vis-ellipsometry and broadband dielectric spectroscopy (BDS) are combined to study the glassy dynamics of thin (>= 10 nm) layers of atactic poly(methyl methacrylate) (PM MA) annealed and measured under identical conditions. In order to unravel a possible effect of the underlying substrate, the interfacial interactions are systematically modified ranging from strong attractive interactions for covalently bonded PM MA brushes with high grafting density and for native silicon oxide (Si/SiOx) to weak and strong repulsive interactions as realized by Au-coated and HMDS-treated Si/SiOx surfaces, respectively. Down to the thinnest analyzed PM MA layers and independently from the applied substrate, both methods deliver-within the experimental accuracy (+/- 1 K for BDS and +/- 2 K for ellipsometry)-a coinciding result. The glassy dynamics arc not altered due to the one-dimensional confinement in these thin polymer layers. The results are discussed with respect to the highly controversial literature and the impact of the preparative conditions.