Vijnana Bhairava Tantra
86: When the yogi knows the three states of consciousness, ie., visva (waking) in which there is limited knowledge productive of duality, taijas (dream) in which there is perception of the impressions of the exterior, prajna (deep sleep) in which it is all darkness, as the form of Bhairava (bhairava rupam), he is then filled with the splendour of infinite consciousness.
87: In the same way, at dark night, by contemplating for long over the terrible circumambient darkness, the yogi will attain the nature of Bhairava.
88: Similarly, while closing the eyes, one should contemplate on the profound darkness spreading in front as the form of bhairava. Thus he becomes one with that.
89: When some organ of sense is obstructed in its function by some external cause or in the natural course or by self-imposed device, then the aspirant becomes introverted, his mind is absorbed in a void that transcends all duality and there itself his essential Self is revealed.
90: If one recites the letter A without bindu or visarga then, O goddess, Paramesvara, a magnificent torrent of wisdom appears suddenly.