Because you cant see a damn thing! YES! 

This also goes for other dark places in game, like the other vaults. And from my discovery of watching Fallout videos on The Tube most of the screens of the video at night looks completly different, like it is way over saturated.  So I went into my settings to see if i could change the video display to something more visable. 

All I saw was "Brightness" and" "Menu Tranceparancy".....No contrast, no colour, or sharpness...Just god damn brightness!

So I fiddle around with the brightness and I either get one that makes good atmosphere but bad visablity especialy at night, or an over staturated, bright but visable setting that makes the darkness parts alright I guess. 

Come to think of it..I had the same damn problem in Bioshock and Oblivion in dungeons. So tell me how you dealt with this and what settings you used. Cheers