Explanation:

Shankara stated that the self in the deep sleep is nirguna i.e. not associated with any gunas or qualities. Though the word guna means property of any entity in general through yoga (root meaning), but the word guna is fixed in usage in the properties of awareness only like seeing, enjoying, hearing, fear, courage, generosity etc. The word guna is used in the three qualities i.e. sattvam, rajas and tamas. These three qualities are confined to the properties of awareness only as we see in the chapter of Triguna Vibhaga of the Gita. The property of burning of fire is indicated by the word dharma and not guna. Therefore, the absence of any guna in deep sleep means the absence of all the characteristics of awareness. In conclusion, it means the absence of awareness itself! When the awareness disappeared, the remaining item is only the basic form of awareness which is inert energy lacking all the gunas. Hence, Shankara referred the inert energy only through the word Nirgunam in the deep sleep. Such inert energy is qualitatively one and the same cosmic energy. The inert energy remaining in the deep sleep quantitatively multiplied by infinity becomes the cosmic energy. The cosmic energy being the creator, ruler and destroyer of the world is the greatest or Brahman (Here Brahman is not in the sense of God). Therefore, the self is qualitatively the same Brahman. The difference between the self (Atman) and Brahman (cosmic energy) is only quantitative due to the difference in the media of human body and the cosmos. Thus the difference between Atman and Brahman is only due to their media (Upadhi bheda) as per the explanation of Shankara.