We study the evolution of the lepton number for a SU(2) doublet consisting of a massive neutrino and a charged lepton. By choosing a specific initial lepton family for a neutrino we can compute the evolution of all lepton family numbers. We study the phenomenology of relativistic and nonrelativistic neutrino physics under this framework. The nonrelativistic region is of particular interest due to the Cosmic Neutrino Background (CνB) predicted from big bang models. In that region we find the expectation value of the lepton family numbers are sensitive to the absolute mass and mass hierarchy of the neutrinos. This is based on the works 2101.07751 [hep-ph], 2106.02783 [hep-ph], and additional results in preparation.