In this paper we study the dynamics of the nighttime lower ionosphere by monitoring daily values of the VLF modal interference distance. This distance depends of the nighttime modes propagating within of Earth-lower ionosphere waveguide, therefore it can provide us information on the behavior of the lower ionosphere in nighttime condition. We have used a long-term database of almost 6 years (2006-2011) from a long transequatorial VLF path. Our results show well defined variability of the nighttime height (hN) in different timescales such as: a) day-to-day variability with maximum variation around 5 km; b) seasonal variability in which hN assumes maximum value around 88.21±0.24, and minimum around 86.89±0.13 km, during north hemisphere winter and summer, respectively. Finally, a preliminary correlation analysis indicates a possible relationship between hN and the long-term solar activity.