Is there a way to sleep in Stalker: SoC without using a mod? I'm by the beds but I don't see an option to use it at night. Also, I've read about a "sleeping bag" but I don't see that in my inventory.

I think sleeping bag is a mod?

So without modding there's no way to sleep in the game?

Correct. There is no sleeping in the base game.

In Shadow of Chernobyl you can't sleep unless you install a mods that allows it like ZRP or AMK.

In Clear Sky also you can't sleep, which is even more annoying because the night are quite pitch black. Use the Complet Mods dude, warmly recommanded for both SoC and Clear Sky :)

There's not even resting in base game?

So if you don't have night vision and and it's pitch black, you just have to go through and hope for the best or wait?

This is resting only in Call Of Pripyat on vanilla version I mean. For the SoC and Clear Sky, you'll need mods.

In the normal version of Shadow of Chernobyl, night is actually very bright. They became very dark in Clear Sky and Call of Pripyat though. Besides, you have a flashlight.

Also nightvision on SoC is horrible. There are two, green and blue but both of them actually just make it harder to see no matter what the situation. Nightvision also became useful in the sequels.

Where do you find the nightvision goggles in SoC?

Not really horrible, it just immitates low gen(early ) nightvision. There is no easy access to gen 3 night vision in Europe or Russia.
And you gotta get used to bad equipment :)

I'd forgotten NV actually existed when playing the base game. Only realized I had it during a modded playthrough after I acquired a suit with second tier night vision, and when I tested it out, it was actually surprisingly helpful; maybe they modded that, also.

Is it correct to say that if you can't sleep then there's no way to advance time or move along the day/night cycle other than by wondering around? For example, if I want to sneak into the army base at night - I'd just have to run laps around until the sky changes?

Pretty much, yeah, although I always just blew my way through during the day.