Where is the makor for not going to a Beis HaKavoros by night?
Answer:
It is permitted in principle to go to a cemetery at night. Indeed, it is told of bachurim who wished to avoid the draft in Europe that spend several nights sleeping in the cemetery to ensure the weren’t caught.
However, based on kabbalah, and for reasons similar to the idea of not learning Torah Shebichtav, some write that one should not visit a cemetery in the first half of the night (before chatzos). The Gemara (Berachos 18b) indicates that a cemetary is a place where the spirits of the dead are present, in particular at night (see Rashi), and therefore there is reason to avoid the place.
See sefer Divrei Torah (Vol. 3, no. 44), as cited by Menuchas Emes (Gross, p. 136).
