B&w emphasizes certain things and de-emphasizes others. I think it emphasizes contours, geometry, and texture. Abstract shapes and connections are sometimes more clear in b&w than colour.
I think there is some relevant physiology to consider. We have two kinds of vision: cone vision is "colour" and used primarily when there is plentiful light, and the other (rod vision) is much more dehued, and provides less accutance but spectacular sensitivity. I think this is important to bear in mind because it means that we might naturally associate dehued, "softer" subjects with lower light.
Because of this physiological issue, perhaps b&w is more connected in our thinking to abstract shapes and feelings, as opposed to vivid detail and hard edges. What are the things we notice at night? We don't search for tiny details so much; instead we notice shapes and light vs. shadow and such. So the brain might actually be trained -via the rod/cone mechanism- to interpret b&w photographs in a different way. B&w images may connect more naturally to the parts of our thinking that try to assimilate disparate elements of a scene, to make intuitive connections.
In other words, when there isn't much light/information around, our brains probably rely on intuition to connect the dots. In stark contrast, colour cues are probably more associated with direct information that requires very little abstract thinking, e.g. a big red stop sign!
Because of the low light issue, I do think that b&w images can intrinsically connect a certain feeling of dreaminess. Not all the time of course, but as a thought experiment, just imagine a b&w image of a sleeping child versus a coloured one....