"That which is not present in deep, dreamless sleep is not real.”
- Ramana Maharshi, States of Consciousness.
Apart from the three states of sleep, wakefulness and dreaming, there exists a fourth state or turiya. The Mandukya Upanishad defines turiya as:
"The fourth (i.e. turiya) is NOT a state. It is the background on which dream and wake arises and disappears. Turiya is just another term to describe pure awareness. It is also called the Nirvikalpa."
A similar explanation is found in the Key to Theosophy, 165, "After the dissolution of the body, there commences for it a period of full awakened consciousness, or a state of chaotic dreams, or an utterly dreamless sleep undistinguishable from annihilation, ... These are the post-mortem fruits of the tree of life. Naturally, our belief or unbelief in ... that immortality as the property of independent or separate entities, cannot fail to give colour to that fact in its application to each of these entities.”
Certain yogic practices talk about "being asleep while awake and awake while asleep" - an enlightened person in the waking state is unaware of the Ego and the illusions of the memories and desires, and when sleeping is still aware of the Divine and does not sink into self-oblivion.
"Then, as you pass into deep, dreamless sleep, you still remain conscious, but now you are aware of nothing but vast pure emptiness, with no content whatsoever. But 'aware of' is not quite right, since there is no duality here. It's more like, there is simply pure consciousness itself, without qualities or contents or subjects or objects, a vast pure emptiness that is not 'nothing' but is still unqualifiable."
- Ken Wilber
Siddhartha’s deep sleep and awakening symbolizes the end of the Ego, and the stirring of the consciousness to a higher frequency that is in tune with Nature, and the Universe.
Deep and Dreamless Sleep is a 2006 movie that retells Dante's Inferno as a New York City love story.