The distinction between dream and deep sleep is admitted by modern physiology, not only by our own internal experience. So the understanding of cause in Indian philosophy is, in one of the schools of Indian philosophy, it is called Purinama Vada. That means the cause is transformed into the effect. Before the distinct features of the effect are visible, you are calling it a cause. When the names and forms are distinct, you call it an effect.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 1-5.