Deep sleep is a phenomenon recognized in every culture of the word. Modern physiology also shows the different brain states associated with waking, dreaming and deep sleep. The moment the mind shuts up, feels like nothing blank. And yet it can't be so the blank itself was, uh, revealed to something.
Swami Sarvapriyananda teaches the Katha Upanishad, one of India's most revered texts which is also a part of the Vedas. It is a collection of philosophical poems representing a conversation between the sage Naciketas and Yama (god of death). They discuss the nature of Atman, Brahman and Moksha(liberation).