Analytically, numerically and experimentally the effect of the material dispersion of the dielectric on the oscillation period of the electric field in a single cycle wave packet (a light bullet) is investigated. Wave packet oscillates due to periodic phase shift between the envelope of the pulse and its carrier wave. The influence of nonlinear shift of phase and group velocities on the analytical estimation of the oscillation period of a light bullet with a different carrier wavelength is considered. Keywords: group velocity, phase velocity, material dispersion, absolute phase shift, single-cycle wave packet, filamentation, light bullet.