Drawing on the gyan from dreams and sleep pattern also (purifies the chitta).

There are four states of mind – conscious, unconscious, subconscious and super-conscious. State of dhyan is the state of super-conscious. Dreams pertain to subconscious, which plays before us our various thoughts and desires juxtaposed with things we see or events that occur around us. However at times, certain subtle experiences also happen in a dream state, which is the activation of super conscious. By studying these various states of mind through practice, being able to differentiate between them and hence, gaining control over them, the chitta gets purified.