In his dialogue 'The Ending of Fear', Francis Lucille talks about our experience of deep sleep. He says we tend to think of it as a blank. Indeed I suppose we regard it as one step removed from 'unconsciousness' except that we know that we can be awoken if someone calls our name and therefore there must be something still aware. The reason for this is that there is no 'objective' knowledge, and this is the only kind of knowledge available to us in the waking state. In order to clarify, he differentiates between the actual condition of being in deep sleep, which he calls 'deep sleep' and our perception of it (attitude/viewpoint) from the waking state, which he calls the 'deep sleep state'.
Clearly we can only actually talk about the 'deep sleep state' - the experience of 'deep sleep' itself is not available, by definition, to the waking person. Thus our attitude that it is a 'blank' or an 'unconscious' state is a view entirely of the waking mind. In fact, claims Lucille, deep sleep is our true nature. It is the same condition as that which exists between thoughts when the mind is still. It is not a 'state' as such but the background to all other states (turiya).
He uses the analogy of faces carved in stone. Our natural tendency is just to see the faces but if we look closely, we see the stone in between. Subsequently, it is possible for us to look at the faces and see that they are stone too. So it is with the three states of which we talk in our waking state viz waking, dreaming and deep sleep states - these are the faces. Deep sleep (turiya) is that which underlies all of our experience - the 'stone' - and indeed this is all that there is, in fact.
This entire analogy is like the Buddhist metaphor of rivers and mountains. Initially, rivers are just rivers and mountains are just mountains. Then you embark upon a path towards truth and get embroiled in philosophical circling thoughts and rivers cease to be merely rivers and mountains cease to be mountains. Finally you are 'enlightened' and they are once again what they always were, forms imposed upon the single reality. Similarly, in the metaphor of the stone faces, having 'woken up' we can see the faces (rivers and mountains) once again but we now know it to be, in reality, only stone (turiya).
Thus, to recap, deep sleep is the condition underlying all of the other states and, once appreciated, is seen to be, in effect, all there is. Lucille says "The actual experience of deep sleep is turiya, consciousness knowing itself by itself, a timeless non-experience. Once this is recognised, one will also recognise that the actual experience of anything, including any state, is also turiya." Deep sleep is "the ultimate witness in the absence of any witnessed mentations." We might think therefore that there are four states, viz. waking, dreaming, deep sleep and turiya but Lucille says "There is an ambiguity here which originates from the confusion between the levels of illusion (waking, dreaming and sleeping) and the level of reality (turiya). Since illusion and reality never co-exist, there were never four states in the first place, but three (illusory ones), from the vantage point of an illusory personal entity, or one (reality), from the vantage point of consciousness. In reality, there is only one state, turiya. Everything else is illusion."
Dreams are frequently examined for all sorts of reasons but their real value is as an analogy. This state of turiya that has been mentioned is to our normal waking state as the waking state is our dream state. Whilst you are dreaming, your dream ego is quite convinced that your dream is the reality. If a dream character in your dream told you that you could 'wake up' and discover that all this was an illusion, your dream ego would not believe it. Our waking state is effectively the same.
Enlightenment takes us to the background awareness of turiya, where the delusions of the waking state are recognised as effectively a dream. Of course, once you have woken up, you are able to say 'but that was only a dream - it's not like this reality here now'. But you can say exactly the same about our current waking state. Once we are enlightened, we can look back on the waking state and see that it was only effectively a 'dream'.
There is no 'ocean consciousness' with lots of 'individual consciousnesses' that are either somehow a part of it or that periodically connect with it or dip into it. There is only Brahman - nothing separate from it and no 'parts'. Since there are no 'things', there is no thing that could not be conscious (or that could be, if it comes to that!). The essence of the theory is extremely simple; it's just that as soon as we try to understand it, we start to make it all very complicated!
We use and investigate all of these ideas in order to reach a position where we accept the conclusions intellectually - as far as they go. They can never go all the way because reality is forever beyond conceptualisation and logic. Once you reach that point, you have to just drop all of this stuff and allow what has been absorbed to take on the status of faith until it transforms into a more intuitive acceptance. From there, you just "wait without hope, for hope would be hope for the wrong thing" (T.S. Eliot) until, just possibly, eventually, realisation dawns.