Err... I was under the impression that BIS had worked in a realism algorithm....
NVG is much less effective at dawn/dusk due to the dodgey light (as in real life), and slightly less effective when below a certain threshold of ambient light is available.
I've found that NVG will work very well in the middle of the night, but will start to get very flakey towards dawn/dusk. This is how it should be as far as I know...