Chandogya Upanishad:- "When a person is sleeping soundly, free from dreams, with a still mind, that is the Self, fearless and deathless. That is Brahman, the supreme."
One has to find answers for: - What is that becomes waking? What is it that becomes dream? What is that becomes deep sleep. As one goes in deeper self-search he becomes aware of the fact that the consciousness alone is the cause of the three states and it itself is uncaused. Until one thinks the self is limited to an individual and it is within the heart he will never be able to come out of physicality. The self is not an individual because it pervades in all the three states and it is formless substance and witness of the here states. The individual experiences of birth, life, death and the world are nothing to do with the self because the self is birth -less and deathless. The form, time and space are nothing to do with the self, because it is formless, timeless and space less existence. Thus judging the truth on the bases of form, time and space is erroneous. Thus ‘I’ or ‘I AM’, which is limited to physicality, will yield only half truth.
That is why Sri, Sankara says:- VC-63. Without causing the objective universe to vanish and without knowing the truth of the Self how is one to achieve Liberation by the mere utterance of the word Brahman? — It would result merely in an effort of speech.
65. As a treasure hidden underground requires (for its extraction) competent instruction, excavation, the removal of stones and other such things lying above it and (finally) grasping, but never comes out by being (merely) called out by name, so the transparent Truth of the self, which is hidden by Maya and its effects, is to be attained through the instructions of a knower of Brahman, followed by reflection, meditation and so forth, but not through perverted arguments.
The individual experience within the dream is reality with in the dream. Therefore waking reality is also is reality within the waking. This waking is parallel dream and dream is parallel waking. Only when self-wakes up in waking the dream becomes unreal. When the self-consciously wakes up in its formless nondual true nature in the midst of waking experience then all the three states are mere mirage created out of its formless nondual true nature.
The form time and space within the dream are falsehood when waking takes place. Similarly the form, time and space of the waking are also falsehood when wisdom dawns. The wisdom dawns only when one realize the self is neither the waking entity nor the dream entity. Thus it is erroneous to judge and reason and conclude the ultimate truth on the base of waking entity and waking world because waking experience itself is illusion.
Suppose you were holding the same view in the dream, all your experiences and views are reality within the dream. The dream becomes unreal when waking takes place. Similarly the waking becomes unreal when you become aware the fact that, the self is not an individual. Thus whatever individual experiences experienced within the waking is as unreal as dream.