I was placing torches, glowstone and lava next to half slabs. After doing this I wanted to check the light levels in F3. I noticed the lightlevels stayed at 0. when i jumped though they changed to the actual light levels and then went back to 0

What I expected to happen was...:
The light levels would change while walking around

What actually happened was...:
Sky and Block light levels stayed at 0. they only changed when jumping

Steps to Reproduce:
1. place a few slabs and light sources
2. press F3
3: walk around and look at the light levels
4: jump and look at the light levels