We develop a microscopic model to describe the observed temporal fluctuations of the fluorescence lifetime of single molecules embedded in a polymer at room temperature. The model represents the fluorescent probe and the macromolecular matrix on the sites of a cubic lattice and introduces voids in the matrix to account for its mobility We generalize Lorentz's approach to dielectrics by considering three domains of electrostatic interaction of the probe molecule with its nanoenvironment: 1) the probe molecule with its elongated shape and its specific polarizability 2) the first few solvent shells with their discrete structure and their inhomogeneity 3) the remainder of the solvent at larger distances, treated as a continuous dielectric. The model is validated by comparing its outcome for homogeneous systems with those of existing theories. When realistic inhomogeneities ore introduced, the model correctly explains the observed fluctuations of the lifetimes of single molecules. Such a comparison is only possible with single-molecule observations, which provide a new access to local field effects.