Advaita Vedanta says just take the experience which you are having already. If you dream and if you fall asleep then you are already in central part, he writes. Upanishad will come and say so this is what is called the Waker and the Waking world. This is the experience of deep sleep for those who want to know more about it.
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).