I have no hostile mobs in my world. The last thing of note I did was to apply a Looting enchantment (from a book) to my sword. I was hoping to try it out that night but nothing appears.

I still see hostiles in the Nether, so not problem there.

I've checked the difficulty and it was still on Normal. Switched to Hard and back and still no hostiles. 

Edit: Windows 10 Pro, i9-9900, 64GB, 2xGTX1080.