Hostile mobs have apparently stopped spawning on my single-player world; and no, I'm not on peaceful, don't even bother asking. They were spawning normally up until a short while ago; however, not only are none spawning on the surface at night now, but I don't seem to be encountering any underground either. The mods I have installed are as follows:
Ars Magica 2 1.1.2b
Antique Atlas 2.1.4
BiblioCraft 1.5.4
Thaumcraft 4.05b
Thaumic Tinkerer 2.0