On the mandukya upanishad and the state of deep sleep

Question

The question is how do we know that turiyam persists in the deep sleep state?

The Mandukya Upanishad suggests that we inhabit 3 identifiable sources of consciousness that is namely and a 4th one simply called Turiyam meaning the fourth which persists in all of these 3 variable states OR is the only persistence within these three:

1. Waking state
2. Dream state
3. Deep sleep state

Vedanta seems to hold the position that that which changes is not the ultimate reality and it makes intuitive sense to me. I also understand how consciousness or the watcher is present in the dream state and the waking state.

I always have an issue with the deep sleep part. Let us assume consciousness is not the permanent reality and an epiphenomenon of the brain just for the sake of argument. Then during deep sleep, this consciousness is "turned off" simply; and it is the memory of lost time that enables us to account for that lost time in deep sleep, after we have entered the waking consciousness. Similarly for general anesthesia when drugs simply stop all brain activity associated with experiencing anything. Why should anyone assume then that turiyam persists in the deep sleep when one cannot comment anything about deep sleep? Sounds like an assumption to me. Saying that consciousness is present in the deep sleep state but simply has nothing to illuminate can also be said as there is a deep sleep experience yet the consciousness itself doesn't exist at that point. Does it make logical sense?

Answer 1

The question is how do we know that turiyam persists in the deep sleep state?

Because you experience happiness during deep sleep.

The medieval Vedantic scholar Ramanujacharya has addressed this in his Sri Bhashya, which is his commentary on Vyasa's Brahma Sutras.

We now come to the question as to the nature of deep sleep. In deep sleep the quality of darkness prevails in the mind and there is no consciousness of outward things, and thus there is no distinct and clear presentation of the 'I'; but all the same the Self somehow presents itself up to the time of waking in the one form of the 'I,' and the latter cannot therefore be said to be absent. ...what he thinks [after waking up from sleep] is only 'I slept well.' From this form of reflection it appears that even during sleep the Self. i.e. the 'I,' was a knowing subject and perceptive of pleasure.

In our life we see that people love to sleep a lot, take naps, sleep in late, etc. We also see that people want to get a good night's sleep, and sometimes they wake up fully rested, and at other times they didn't get good sleep and wake up irritated.

This is because you experience the bliss of the Jivatma during sleep.

This state is identical to the state of Kaivalya, which is a type of after-life where the Jivatma experiences its own pleasure. It is like experiencing deep sleep for millions of years.

However, this state is temporary and the happiness experienced here is inferior to the happiness experienced in the state of moksha, or union with Brahman.

Indra in the Chhandogya Upanishad criticizes the inferior state of Kaivalya:

VIII-xi-2: He came back again, fuel in hand. Prajapati asked him, 'Desiring what, O Indra, have you come back, since you went away satisfied in your heart?' He replied, 'Revered sir, in truth this one does not know himself as "I am he", nor indeed these beings. It seems as if he has gone to annihilation. I see no good in this'.