I find very difficult to believe that sunlight can reach sewers.
So i would suggest to put a constant perception malus in them.
A perception malus like when it's night, cold, and you are alone, and you seek for shelter, in the woods for example, or in a manor haunted by satanist squirels who want to devour your soul because they misread the word "soul" and think it's another word for "peanuts"...