  We study the space-time evolution of ``flavor'' neutrino wave-packets at
finite temperature and density in the early Universe prior to BBN. We implement
non-equilibrium field theory methods and linear response to study the
space-time evolution directly from the effective Dirac equation in the medium.
There is a rich hierarchy of time scales associated with transverse and
longitudinal dispersion and coherence. A phenomenon of ``freezing of
coherence'' is a result of a competition between longitudinal dispersion and
the separation of wave-packets of propagating modes in the medium. Near a
resonance the coherence and oscillation time scales are enhanced by a factor
$1/\sin2\theta$ compared to the vacuum. Collisional relaxation via charged and
neutral currents occurs on time scales much shorter than the coherence time
scale and for small vacuum mixing angle, shorter than the oscillation scale.
Assuming that the momentum spread of the initial wave packet is determined by
the large angle scattering mean free path of charged leptons, we find that the
transverse dispersion time scale is the shortest and is responsible for a large
suppression in both the survival and transition probabilities on time scales
much shorter than the Hubble time. For small mixing angle the oscillation time
scale is \emph{longer} than the collisional relaxation scale. The method also
yields the evolution of right-handed wave packets. Corrections to the
oscillation frequencies emerge from wave-packet structure as well as from the
energy dependence of mixing angles in the medium.
