When standing on Soul Sand in 14w33a while in the f3 menu, the light level will remain 0 no matter the actual light level. I believe this is casue by the fact that light level is based off what block your player's feet are in/on, and when standing on Soul Sand, you are lower than the texture, so it takes the light level from the area the block is in, which would always be 0 because there is always a block in the spot.