According to the Upanishad it is just pure consciousness. Not consciousness dressed up in the subtle body of the dreamer. But quite apart from the causal universe of deep sleep, the subtle universe of dreaming and the gross or physical universe of waking. None of which are anything different from consciousness. They are all dresses projected by consciousness which consciousness puts on and acts out these experiences. This is something that's not known to us. We never think about ourselves as these. Our whole concept of ourselves is heavily based on the waker. And yet our sense of self goes on.