In the last mantra in Mantra V, we were introduced to the individual the sleeper. The Upanishads defined deep sleep as a state where one does not sense any desire or gross objects. They also said it is an experience of non-duality and that nothing is known per se.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 1-5.