Nirvikalpa Samadhi is unquestionably the same as deep sleep.+

Nirvikalpa Samadhi and deep sleep are the same from the standpoint of the Soul, the innermost ‘‘Self’’ the absence of the known. The knower was there.

How does Samadhi give Gnana? Only by preparing yourself to see that the world disappears and re-appears and, that non-duality is here and duality there, to convince the man that in non-duality one won’t disappear as he doesn't disappear in Samadhi or deep sleep. Another advantage of Samadhi is one gets the capacity to forget the external world and to treat it as an idea.

Yoga can never give you the fundamental thing, that the world is an illusion. Only Gnana can give it. Nirvikalpa Samadhi is unquestionably the same as deep sleep, and all ideas are refunded back there too. One must learn what ideas are when all the ideas of the universe-existence go back into one’s mind through Yoga. Then one learns this. How has he to learn that whole universe is consciousness or Brahman if he stops at Nirvikalpa Samadhi? Without perceiving the universe, and having a duality before him, it is impossible.

Nirvikalpa has no duality, hence it cannot tell you about the universe. The yogi who emerging from Samadhi and says he found Gnana there, says it to a second person, hence there is duality again. If he were a real Gnani, there would be nobody for him to tell that he had experienced Gnana.

‘Self’-knowledge will interest only a few people; the rest are interested in Religion, yoga other paths and pleasure hunting.

Bhagavan Buddha gave up his austerities of yoga as impossible and useless. (Page.70/71 "Buddhism in Translation” by Warren)

Bhagavan Buddha got enlightenment only after he gave up Yoga. Unless one exercises his reason--there is no chance of getting the truth.

When the yogi enters this highest Nirvikalpa (effortless) Samadhi, he will at once enter deep sleep. This will make plain to him after he wakes, that the inner ‘Self’ he sought and found, the Atman, is reached only when all his ideas are refunded into it, when there is then all the features of non-duality, one without a second. However, the yogi must later wake up, emerge from Samadhi and there is duality again, for the world of objects confronts him. So now he has to work on the next stage which is to find consciously in the waking experience the same non-duality that he unconsciously knew in sleep. This is done by learning that the universe is an object for the formless subject, and then refunding the universe ~idea back into its source, which is the Soul, which is present in the form of the consciousness. Only at this final stage, he says "Atman, which is present in the form of the consciousness, is the same as the Brahman. Brahman is the ultimate truth.  Now he is fully aware of it.

All yogic visions, however, wonderful will pass away; they go as they come. They have the value of dreams. They are not the truth which is un-passing and beyond change.

One can’t shut his eye to the universe, which confronts him as in Samadhi of yoga and see supreme reality. One can know it only by keeping himself’ clear and open.

Sage Sankara says:~ ‘The yogi must add discrimination to his quest. : ~ Santthosh Kumaar