Mobs won't spawn

Going through the first night in a new survival in the biome "Sunflower Plains", I noticed that hostile mobs were not spawning, even though the game was set to Easy. Also, I noticed that through the middle of the night, the light level coming from the sky was marked as "15" in the F3 debug screen.

EDIT: My render distance was set to "6 chunks", and through a new world in the Extreme Hills biome, I found 1 creeper throughout several minutes of waiting.