I did notice that time was passing - with the lighting changing. One other suggestion would be that the the colour palette is tinted to be more nighttime colours (rather than just going darker - which I think is what happened).
One good ref for this is Cult of the Lamb - the camp has a day/night cycle which tints the palette to be more blue/purple. Perhaps you could also have some torches or something around the level that become lit at night to emphasise the feeling.