the light will not do anything but mess up the hops, just like with any beer. you also likely will not get sourness with brett, but should get some funk. also, i would keep the bottle covered (if it is actually a sealed bottle) because brett is a hungry bugger, and will chew through more sugar. if it eats too much you may have a bottle bomb on your hand. I would keep it covered anyway, the light is not going to help the brett do its thing.