In the Brahmasutra Shankara Charya considers this, asking why only dreaming and samadhi can be a mystical experience. But she is talking about what the Sufi said, that also different schools of Hinduism say there's a difference between dreams and mystic experiences. "Here we are not interested in the object. Everything is experienced by consciousness here," he says.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 1-5.