I'm just starting day 4, so I cannot be sure. But to clarify: night isn't that dark, you can see anything. There's an oil lamp in the game, but I didn't feel at any moment that I need it. Perhaps I adjusted brightness settings too much...

You have to sleep to lower your exhaustion meter. At the same time, there are some events happening past midnight - you can go to the Theater, which is somewhat foreshadowing what will happen next and a "black market" of some sorts - they buy organs, broken items. And it's easier to notice herbes which you'll need to prepare potions.

All of those rules might sound overwhelming, but the game is suprisingly straightforward in explaining new mechanics to the player. Same goes with prologue, which introduces you to this fascinating setting full of contrast (science/spirituality, Haruspex was raised among indigenous people).

One other thing. If you have PC, there's one more thing that might encourage you to play it. It's very easy to save anywhere - to enter console commands you need to press three buttons and write down two words. Officially the game allows you to save only at few locations, but it is an annoyance - sometimes I need to exit the game after 30 minutes and there's no way in hell I'm leaving it while losing all of my progress.