Deep sleep experience can be taken as illustration for non-duality.

Deep sleep experience  can be taken  as illustration for non-duality.  However, sleep is not the goal.  deep sleep is not wisdom.  It is given as analogy to show that the duality or mind disappears in deep sleep. 

The freedom from duality is possible in this very moment, here and now if the Seeker realizes the fact that, the self is neither the waking entity nor the self is dream entity but the self is the formless soul or consciousness. Formless soul or consciousness is the formless substance, the witness and also the source  of  the mind or universe, which rises as waking or dream and subsides as deep sleep.

  The formless substance and the witness and also the source are one in essence.  Thus there is no second thing exists other than consciousness.  Thus consciousness alone is real because the diversity is mere mirage created out of consciousness. 

Seeker has to train himself by constant reflecting on the formless substance and witness in order to rectify his reasoning base from form to formless.  All doubts and confusion created by duality vanishes when one is able to view the worldview from the ultimate standpoint.