Shankara chari is asked a similar question about existence in the second chapter of the pagavad gita, sixteenth verse. Thente sankra sada says, isness is there, but it's not manifest. Exactly like that, consciousness in deep sleep is there. This is very metaphysical proof. Every culture in the world speaks of deep sleep as an experience. Otherwise you wouldn't.
Inspired by the 15th chapter of the Ashtavakra Gita, Swami Sarvapriyananda gives a discourse on the nature of the Self.