Variation of plasma irregularity spectrum in the lower ionosphere caused by diurnal change in the ionospheric parameters is considered. It is shown that under the constant intensity of turbulent mixing near an altitude of 95 km, the expected fluctuation level of plasma irregularities associated with neutral turbulence is smaller during day-time than at night. The irregularity spectrum that can be measured in experiments must be changed too, namely, the spectral slope is expected to be larger at night.