http://en.wikipedia.org/wiki/Yoga-nidra
Finally, the swami entered the state of (unconscious) deep sleep, as verified by the emergence of the characteristic pattern of slow rhythm delta waves. However, he remained perfectly aware throughout the entire experimental period. He later recalled the various events which had occurred in the laboratory during the experiment, including all the questions that one of the scientists had asked him during the period of deep delta wave sleep, while his body lay snoring quietly.
Such remarkable mastery over the fluctuating patterns of consciousness had not previously been demonstrated under strict laboratory conditions. The capacity to remain consciously aware while producing delta waves and experiencing deep sleep is one of the indications of the superconscious state (turiya). This is the ultimate state of yoga nidra in which the conventional barriers between waking, dreaming and deep sleep are lifted, revealing the simultaneous operation of the conscious, subconscious and unconscious mind. The result is a single, enlightened state of consciousness and a perfectly integrated and relaxed personality.