Consider what happens when we dream. What is that? — Joey
The same internal simulation model of reality as when we are awake still operates when we night dream, which is why things look about the same, except for some occasional flaws, but not anything is coming in externally, such as e/m photons bringing information for internally created vision and light, molecules for internal created odors, molecules for internally created taste, air vibrations for internally created sound.
As there is no light, sound, taste, and smell in the external world when no one is around to sense it, it is just the opposite in a night dream, there being light, sound, taste, and smell only, without the externals of e/m and air vibrations and molecules.
Although a part of the brain is asleep, the rest runs on through imagination via memories, associations, and some outright noise turning into something, along with long term memories being recorded and who knows what else.
All waking life is not a dream or consciousness floating around by itself. The intricate brain and body is not just for show, as in a night dream, and senses and instruments really do take things in.