Are lightning storms more common at night?
The answer is dependent on the geographical location. Over sea, there is hardly any diurnal cycle of lightning (this means that there is no time of the day when lightning is more/less often observed).
In continental regions (e.g. Siberia), however, there is a strong diurnal cycle of lightning: it is most common in the late afternoon, and least common in the early morning.
The third type of regions are regions mainly affected by mesoscale systems (as is the Great Lake region in the US): here, there is also a strong diurnal cycle, but the peak (most lightning) is shifted towards the late evening or early morning. Thus, to answer your question: in these regions, there is indeed more lightning at night.

Reference:
Blakeslee, Richard J., et al. "Seasonal variations in the lightning diurnal cycle and implications for the global electric circuit." Atmospheric research 135 (2014): 228-243.