In dark areas at night and during the day, no mobs spawn, or at least it is rare that mobs spawn. I saw one mob recently it was one of those baby zombies, but that was it. I am using vanilla with no mods.

Steps to Reproduce:... Im not sure how it happened exactly but this is what went down for me.
1. Make Amplified world
2. (probably a few days later) Now im not sure when it was or how far into creating this world it was, but I went outside and everything was foggy, there was no sun or moon, I went to sleep in my bed and the problem seemed solved.
3. From then on I noticed that there were significantly less amounts of mobs. As i said before, it is rare that I see a mob. Some times if i go to lower levels I lll see a creeper. Recently I saw a baby zombie at the height I ussually play at.