From Shankara’s Vivekachudamani:
170. In dreams, when there is no actual contact with the external world, the mind alone creates the whole universe consisting of the experiencer etc. Similarly in the waking state also; there is no difference. Therefore all this (phenomenal universe) is the projection of the mind.
171. In dreamless sleep, when the mind is reduced to its causal state, there exists nothing (for the person asleep), as is evident from universal experience. Hence man’s relative existence is simply the creation of his mind, and has no objective reality.