If you have a light source and you reload the world, it sometimes will only display light in its chunk.

Here's the easiest way to see this:
Make a superflat world, set time to 18000, place a Redstone lamp and power it with a lever (because Redstone torches make light and buttons don't constantly power them) and reload the world by saving and quitting and starting the world again.

See the screenshots below for examples.
I have tried torches, Jack-O-Lanterns, and Redstone Lamps.
Redstone Lamps seem to do this more often than torches.
EDIT: You can't be near the light source if you try to reproduce it. Try flying like I am in the screenshots.
Redstone torches don't seem to do it.
If it doesn't happen on the first try, then do a different world. I only got it to happen in a few of my worlds. They were both superflat.

Seems slightly rare but does occur. Also there is no definite way to reproduce this except to keep trying. It will happen eventually.