Put the summary of the bug you're having here

What I expected to happen was...: 

I expected the sky light level to drop down to 0 at night time, allowing hostile mobs to spawn

What actually happened was...:
It stayed at light level 15 and the only hostile mobs that spawned spawned under trees

Steps to Reproduce:
1. Wait till night time
2. Check F3 debug screen for sky light level and it should be at 15