On a full moon in a swamp at night looking for slimes I noticed no hostile mobs were spawning- I was on normal. In an area without light I pressed F3 to check the light levels: Sky light was at 15, and block light was 0.
Environment
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_60, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 2703876864 bytes (2578 MB) / 3087007744 bytes (2944 MB) up to 3087007744 bytes (2944 MB)
WAI. MC-10410