If you are seeing a "brightening," then I would check Ambient Light, in the Lighting Effect. It defaults to 20%.
Most aspects of Lighting can be animated via Keyframes - look for the "stopwatches."
I'm using Lighting Effects in Premiere Pro CS6 for the first time and am experiencing the same thing rmartin215 describes in his original post.
I noticed after keyframing the Light Type from Spotlight to None, that the overall clip is left darker than when I started. If I toggle the Lighting Effects off, the clip gets bright again -- like the unaltered. Toggle the effect on, and the clip is overall darker.
Bill Hunt's response suggests checking out the Ambient Light parameter, which does default to 20%. When I increase the Ambient Light the clip does brighten.