The Advaita-Vedanta fascination with Deep Sleep

Advaita Vedanta (Nondualist Hinduism) has a special fascination for the state of deep sleep.  That is because nondualist Hinduism holds that the observed (reality) is only created due to the observer.  Since in deep sleep, nothing is being observed, it is claimed that deep sleep is a spiritually higher state pointing to the "pure" observer.

Some quotations will illustrate:

Question: Sushupti [deep sleep] is often characterised as the state of ignorance. 

Bhagavan (Ramana Mahrishi): No, it is the pure state. There is full awareness in it and total ignorance in the waking state. It is said to be ajnana [ignorance] only in relation to the false jnana prevalent in jagrat [the waking state]. Really speaking jagrat [the waking state] is ajnana [ignorance] and sushupti [the sleep state] prajnana [wisdom]. If sushupti is not the real state where does the intense peace come from to the sleeper? It is everybody’s experience that nothing in jagrat can compare with the bliss and well-being derived from deep sleep, when the mind and the senses are absent. What does it all mean? It means that bliss comes only from inside ourselves and that it is most intense when we are free from thoughts and perceptions, which create the world and the body, that is, when we are in our pure being, which is Brahman, the Self. In other words, the being alone is bliss and the mental superimpositions are ignorance and, therefore, the cause of misery. That is why samadhi is also described as sushupti in jagrat [sleep in the waking state]; the blissful pure being which prevails in deep sleep is experienced in jagrat, when the mind and the senses are fully alert but inactive. (Guru Ramana, pp. 112-13)

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Sushupti is the natural state.  Immutability is the true condition of things, for that is independent of external forces. Modification is not the true state, as it is dependent on external causes. . . . The perception in waking and dreaming moments is a modification of the original state. That state of a thing which is independent of external causes is its true condition, and that state of a thing which is dependent upon external causes is not its true condition ; for this state cannot subsist in the absence of the external cause. Therefore, sushupti being the natural condition, there is no modification there, as in waking or dreaming’ (Sankara's Commentary on Taittiriya Upanishad 11.8.1.) 

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When it {the individual soul) becomes fast asleep—when it does not know anything—it comes back along the 72000 nerves called Hita, which extend from the heart to the pericardium, and remains in the pericardium. As a baby, or an Emperor, or a noble Brahmin lives, having attained the acme of bliss, so does it remain. (Brihadaranyaka. 2.1.19) 

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(The teacher Prajapati said) ‘ That which is in deep sleep, at perfect rest, seeing no dreams,—that is the Self, that is the Immortal, the Fearless, that is Brahman.(The pupil) Indra went away satisfied in his heart, but before he reached the Devas (i.e. his home) he saw this difficulty. He thought “ In truth he does not rightly know himself as ‘ this is I ’, nor does he know these beings. Therefore he has reached utter annihilation, and I see no good in this. (Chandogya Upanishad 6.8.1.) 

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This state of profound sleep is not a complete non-being or negative, for such a hypothesis conflicts with the later recollections of a happy repose of sleep. The self continues to exist, though it is bereft of all experiences. The consciousness is continuous. You feel you have existed even during sleep as soon as you are awake. You feel that you exist always. Vedantins build their philosophy around this Sushupti Avastha. This stage gives them the clue to the non-dual state (Advaitic state). A careful study of the three states-Jagrat, Svapna and Sushupti (waking, dreaming and deep sleep)-is of immense practical use for the clear understanding of the Vedanta. (Swami Sivananda)

Let's start from the last excerpt.  Swami Sivananda claims that "The consciousness is continuous." even during deep sleep. That is just factually wrong.  So he can be summarily dismissed.

A lot of mental activity is absent in deep sleep (as evidenced by electrical activity maps by MRI and PET), including the kind of consciousness during our waking state, or even dreaming, that we all understand.  Nondualists claim that that attribute-less state (no objects of consciousness) means that the so-called "pure consciousness" existing during that time is evidence of the Self (with a capital S), the atman.  What they probably mean is that during deep sleep, the "spiritual" consciousness is untied (or dis-identified) from body/brain.

That is just hokum.  By definition, there is no awareness in deep sleep - otherwise it is not "deep" enough.  There is no "pure consciousness" in deep sleep.  There is just oblivion.

Many vedantins who want to challenge this sense of oblivion claim that you do say when you wake up that you had a sound sleep.  And according to them, it is evidence that you were aware that you were in deep sleep.  That is also not true.  When someone claims he had a restful night, he is remarking about an absence of restlessness and the sensations of refreshed-ness when he has just woken up.  It is not a comment about having been aware, it is a comment about having slept so soundly that one was UN-aware.  Deep sleep is known to induce greater restfulness to the brain and the body.  A cursory survey of the scientific literature about REM sleep vs non-REM sleep and its neurochemistry will be instructive for anyone curious.

Sankara is stating that the "true" state of something is that which does not undergo change.  Waking and dreaming consciousness goes through moods and different perceptions, so according to Sankara, the "true" consciousness is that which is not externally directed, or even directed to be aware of the mind.  That is, the experiencer is waking and dreaming states is intermingled due to identification, but the experiencer devoid of experiencing anything but itself (in deep sleep) is the true self.  That could be an interesting philosophical position, if only Sankara could tell us how and why he assumes there is an experiencer in deep sleep.  This is also what yogis term as "nirvikalpa samadhi" (attribute-less union/bliss). 

But it is just an abstract notion.  There is no such thing from all that we know and understand about consciousness.

And at least one man (take his words with a big grain of salt, as he doesn't explain what he experienced and is not very rigorous) who claims to have experienced this condition says that this is no different than deep sleep and oblivion and that it is entirely useless in fostering any understanding.