Deep sleep brings on a sense of non-dualistic peace with it.+

Sage Sankara: ~VC~ if the universe is true, let it then be perceived in the state of deep sleep also. As it is not at all perceived, it must be unreal and false like dreams.   

From the dualistic perspective, people see the world in which they exist as a reality.   They see their body as the body, their ego as the ego, and the world in which they exist as the world whereas a Gnani sees everything from the non-dualistic perspective.  A Gnani sees his body as the consciousness, his ego as consciousness, the world in which he exists as the consciousness. For a Gnani, whatever is seen, known, believed and experienced is nothing but consciousness.  There is unity in diversity in Gnani's understanding whereas the ignorant people see only diversity and separation.

Thus, it is necessary to learn to view and judge the worldview from the non-dualistic perspective to realize that form, time and space are one in essence.   There is nothing to realize other than realizing that consciousness is everything. The consciousness is second to none.  

When everything is nothing but consciousness then what remains is not consciousness. 

Deep sleep brings on a sense of non-dualistic peace with it. This experience one gets only in the absence of the ‘I’.  Therefore,  there is a need to investigate what this ‘I’ is? The ‘I’ appears and disappears.  

A deeper investigation reveals the fact that the ‘I’ is present in the form of the mind. The mind is in the form of the universe. The universe appears as waking or the dream (duality) and disappears as deep sleep (nonduality).  

Self is not the ‘I’. If you hold the Self as ‘I’ then you will never be able to realize the truth, which is beyond the form, time, and space.

The Self is the Soul, which is present in the form of consciousness. The Soul is the witness of the ‘I’ which appears and diapers. Thus the ‘I’ is not permanent.

That is why Bhagavad Gita: ~ The permanent is always there, only the transient ‘I’ comes and goes. (2.18)

The ‘I’ hides the truth of the whole. 

People think the ‘I’ without the body is the Self. The seeker has to understand the fact that ‘I’ is not the Self, but the witness of the ‘I’ is the true Self, which is eternal. 

That is why Ashtavakra Gita 16:10:~ If you desire liberation, but you still say "I," If you feel the ‘Self’ is the ‘I’, You are not a wise man or a seeker. You are simply a man who suffers. 

People are stuck with the reality of the ‘I’, which they take it as real because some Gurus have propagated the Self is the ‘I’. is no need to convince such mindsets. The seeker of truth accepts only the truth nothing but the truth. 

Remember: 

The Mandukya Upanishad too says the unreality of the dream and waking worlds by the term –prapanchopashamam and specifically negates those two states by the terms: naantaH prajnam, na bahishprajnam? -in the crucial seventh mantra that teaches the nature of the Turiya, Brahman, and Atman.

The Upanishad has spoken of the sleep state at length.  This has several purposes?

    1. To reiterate that the objective world does not have a permanent reality

    2. To demonstrate that the subject alone remains as the nondual reality in the sleep state and therefore in all the states.

   3. To prove that the sleep state, being nondual, gives the greatest bliss to the subject consciousness.

   4. To prove to the subject-enquirer that bliss is obtained not from objects but on the contrary, in the absence of objects.

      5. To prove that duality is samsara, misery, and nonduality is liberation, bliss.

       6. To give the sadhaka a? preview? a foretaste,  of the liberated state.

Brihadaranyaka Upanishad 4.3.22 alludes to this experience:-  (In this state a father is no father, a mother no mother, Gods, no Gods, the Vedas, no Vedas.  In this state, a thief is no thief, the killer of a noble Brahmana no killer, a monk no monk, a hermit no hermit. This form of his is untouched by good work and untouched by evil work, for he is then beyond all the woes of his intellect.)

Gaudapada Karika-s of the second, third and fourth chapters are the authority that a mumukshu ought to resort to. Since the reality of duality is born of ignorance even to a just born animal, and therefore the cause and characteristic of samsara, bondage, misery, the Upanishads and the Sages who have followed the Vedic tradition teach the unreality of the duality and redeem the suffering humanity from the misery caused by the duality.

For the sincere spiritual seeker duality (dvaita) and suffering is synonyms. So also Nonduality (Advaita) and bliss (sukha) are synonyms. The Veda, especially the Upanishad portions, is full of teachings directed at establishing the unreality and miserable nature of duality and upholding the non-dual nature of the Atman, the sole reality.

Brihadaranyaka Upanishad 4.3.22 alludes to this experience:- (In this state a father is no father, a mother no mother, Gods no Gods, no Vedas. In this state, a thief is no thief, the killer of a noble Brahmana no killer,  a monk no monk, a hermit no hermit. This form of his is untouched by good work and untouched by evil work, for he is then beyond all the woes of his intellect.).:~Santthosh Kumaar