Actually, the day/night cycles could be incredibly simple, assuming they are using the right game engine. But yes, I LOVE your bed idea. There also must be something special about the night time to make the player WANT to sleep, besides simply health.
True, the night has to have something bad about it.
As for the game engine, this game utilizes Unity (as it states in the opening credits). I have UE4 installed, so I could try to figure out how to add a day/night cycle with that as an experiment, but as far as I know, most programs have confusing or complex ways to implement such cycles. I've never used Unity, though, so I'm not too familiar with its capabilities.
"There also must be something special about the night time to make the player WANT to sleep, besides simply health."
Maybe an Energy meter aside from Hunger and Thirst meters. Eating and drinking can bring it up slightly, but that can be a waste if you're full already, so you'll have to sleep eventually.