It's pretty silly that we have control over that through an "opening hours" setting, but OK.
But you say nighttime is bright already?... Isn't that the whole thing the original poster is complaining about here, that it's too dark to build in?
I don't know about your monitor or settings or anything but mine is calibrated just right and it's pretty dark blue at night with little clarity to build in. I seriously just wait it out every time I wanna build. But I guess I'll manipulate the opening-hours so that it doesn't become dark, if that's how it works.
Why would guests need to see the whole zoo per se to leave?... I mean, yea, that kinda makes sense, like you would in real life, but sometimes parks are too big for one day, which I've definitely experienced, and you leave before having seen everything. - What does it matter to AI?...
I'm of the opinion that they should walk a bit quicker anyway. Not crazy fast, but like 25% faster than currently or whatever.
Anyway, the point is that there's little logic here if they want to have a day/night-cycle and supposed "opening-hours" in the game when it doesn't do anything but make it light and dark. - If there are opening-hours, there would be a period when the park is closed, which is when there should be nobody. But it doesn't go that way in this game.
So it pretends to have day and night, as well as open and closed, but it doesn't reflect in the guest-attendance, they're just perpetually there.
I think they could make more of a fluctuation in attendance, like at night or whatever it could be way less or just thinned out or something.