Sri Aurobindo translates Prashna Upanishad, Fourth Question, Verses 1-2: “Then Gargya of the Solar race asked him: ‘Lord, what are they that slumber in this Existing and what that keep vigil? Who is this god who seeth dreams or whose is this felicity? Into whom do all they vanish?’ To him answered the Rishi Pippalada: ‘O Gargya, as are the rays of the sun in its setting, for they retire and all become one in yonder circle of splendour, but when he riseth again once more they walk abroad, so all the man becometh one in the highest god, even the mind. Then indeed this being seeth not, neither heareth, nor doth he smell, nor taste, nor touch, nor speaketh he aught, nor taketh in or giveth out, nor cometh nor goeth: he feeleth not any felicity. Then they say of him, ‘He sleepeth.’ ‘ ”
The fourth question raises the question of the various states of consciousness, waking, dream, and sleep and the superconscient state of consciousness which resides in the Brahman. When an individual is awake he experiences the material universe through the operation of the senses and the mind. When he sleeps, he does not respond generally to the sense impressions, although it is quite certain that at least the senses of hearing and touch remain somewhat active. This explains why a sleeper can be awakened with an alarm clock or by physical touch or shaking of the body, or by internal pressure for a need for evacuation. Where does the conscious awareness go and what does the sleeper actually experience?
During the dream state, who is awake and experiencing the dreams? We tend to experience dreams as quite real and participate in them and interact with the events in the dreams. We do not, however, tend to remember all dreams, and those that are remembered primarily occur just before waking. Some have developed the skill of “lucid dreaming” where they can be both conscious and dreaming concurrently. With respect to stimuli from the outside world, there does not appear to be a direct link between sensory impressions received by a sleeper and the ability to convert this into a remembered event.
There are different stages of sleep which differ in the depth of withdrawal from response to sensory impressions. REM sleep is a status in which dreaming takes place. Light sleep is closest to the waking conscious state and the sleeper is most responsive to sounds. Deep sleep has the least responsiveness and it is thereby quite clear that the consciousness that identifies with the waking personality is elsewhere.
We may additionally look at states of coma, trance, near death experience, out of body experience, as well as the various states of Samadhi, to help understand what is “conscious” and where that conscious entity goes at times when it withdraws from active interaction with the external world.
The experience of these different states of awareness has been a subject of interest for humanity for many thousands of years. The nature of consciousness, the true inhabitant of the being, and the relation of the mind and the sense organs to the true Existent all come into focus with this review.
Sri Aurobindo, The Upanishads, Prashna Upanishad, pp.297-315