Basically the game mechanics focus on movement speed, and Line of Sight as the primary factors when dealing with stealth. However, does darkness have an effect on this as well? E.g., if I raid a Bandit Camp at night, will I be able to slip by the guards more easily?
It is never mentioned in the game, as far as I'm aware.
I do notice that I get spotted less when travelling at night. Whether that's a case of darkness assisting in stealth, or enemies being dormant at night, I can't say.
However, the latter seems to not be the case. Not only are they machines who do not need sleep (afaik), it's a common videogame trope that night time brings out the enemies, rather than hiding them.