Sri Aurobindo translates Mandukya Upanishad, Verses 5-6: “When one sleepeth and yearneth not with any desire, nor seeth any dream, that is the perfect slumber. He whose place is the perfect slumber, who is become Oneness, who is wisdom gathered into itself, who is made of mere delight, who enjoyeth delight unrelated, to whom conscious mind is the door, Prajna, the Lord of Wisdom, He is the third. This is the Almighty, this is the Omniscient, this is the Inner Soul, this is the Womb of the Universe, this is the Birth and Destruction of creatures.”
We experience, in the waking state, the self-awareness of the ego-consciousness, and we identify with this particular being, form and existence. When we go into the dream state, we continue to attach the experience to this same ego-consciousness, and when we awaken we speak about the dreams we had and what significance they had, and whether they have a direct relation to the experiences of the waking state. When we go into the state of deep sleep, however, we do not carry any awareness of this ego-consciousness either into the state, or back out again. We awaken and do not know where the awareness went during the state of deep sleep, nor have we noted in that state the passage of Time. It is only through a review of references in the waking world that we determine how long we may have been asleep.
Where does the consciousness go when we enter the state of deep sleep? Clearly it does not dissolve entirely, as we return from that state to continue the experienced continuity of the waking life. The Rishis indicate that it goes into a state of Oneness with the Divine, no longer focused on the outer names and forms of the world, but indrawn and concentrated in a state of undifferentiated existence and bliss. The soul is one with this universal and transcendent consciousness and during deep sleep it shifts its standpoint there. The soul returns to the outer forms and outer life renewed and refreshed from its brief sojourn in a status of bliss.
Deep sleep is considered to be a healing and renewing experience and Western medical science agrees that this state is essential to health, immune system action and bodily maintenance with no distractions to draw the energy away. The body is left basically in a mechanical renewal mode while the soul refreshes itself by joining with its own larger existence.
Deep sleep corresponds to the experience of the withdrawal of the sun from our view. Revealed to us then is the timelessness of infinite space and the numberless stars, which are hidden from us when we are immersed in the waking consciousness in the light of day.
Sri Aurobindo, The Upanishads, Mandukya Upanishad, pp.319-321