Teachers such as Osho and Anandmurti Gurumaa define yoga-nidra as a state of conscious deep sleep. One appears to be sleeping but the unconscious mind is functioning at a deeper level: it is sleep with a trace of deep awareness. In normal sleep we lose track of our self but in yoga nidra, while consciousness of the world is dim and relaxation is deep, there remains an inward lucidity and experiences may be absorbed to be recalled later.

..." 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."

I find it deeply healing experience, on both physical and mental level. Enjoy!