Yoga Nidra means Yogic Sleep. It is a state of conscious Deep Sleep. In Meditation, you remain in the Waking state of consciousness, and gently focus the mind, while allowing thought patterns, emotions, sensations, and images to arise and go on. However, in Yoga Nidra, you leave the Waking state, go past the Dreaming state, and go to Deep Sleep, yet remain awake. While Yoga Nidra is a state that is very relaxing, it is also used by Yogis to purify the Samskaras, the deep impressions that are the driving force behind Karma.
There are THREE levels of consciousness: Waking, Dreaming, and Deep Sleep (plus the “fourth” which is the transcendent state known as Turiya). Yoga Nidra is conscious DEEP SLEEP and Deep Sleep is NOT conscious Dreaming. Yoga Nidra is also NOT the transition between Waking and Dreaming. Those are states to explore, but they are NOT Deep Sleep; if it did have dreams, that would be called Dreaming, and would NOT be called Deep Sleep. It is utterly obvious that Deep Sleep does NOT have Dreams to explore. It should be self-evident that Dreaming and Not-Dreaming (i.e., Deep Sleep) are two different things.
Yoga Nidra has been known for thousands of years by the sages and yogis. The state of Yoga Nidra, conscious Deep Sleep, is beyond or subtler than the imagery and mental process of the Waking and Dreaming states. As a state of conscious Deep Sleep, Yoga Nidra is a universal principle, and is not the exclusive domain of any more recent teachers or traditions. Yoga Nidra was taught by the ancient sages for the purpose of exploring the deep impressions or samskaras, which drive our actions or karma. They taught this so that sincere seekers can purify the deeper aspect of the mind-field, which is accessed in the formless state of conscious Deep Sleep.
Meditation: Imagine that you are in the WAKING state, with all of its distractions of mind. You systematically do this and that with directing your attention. You are practicing concentration and letting go, leading you towards meditation. The thoughts and pictures of the DREAMING state start to come to the surface, even though you are still in the WAKING state. Gradually, the distractions fade away, your mind becomes focused, and you are having a nice meditation. You are deeply relaxed. You are still in the WAKING state of consciousness, which is the nature of meditation.
Yoga Nidra: Now, imagine that at some point you DROP or FALL or EMPTY (or other such words) into the STATE of DEEP SLEEP, much like you would normally do at bedtime (when not Dreaming). However, this time, you fell all the way THROUGH the Dreaming state, and came to rest in the DEEP SLEEP state, though, paradoxically, you remained aware and alert. This is the level UNDERNEATH both the WAKING and DREAMING states, and all of the thoughts and impressions that go along with those two states. Now you are in the STATE of CONSCIOUS DEEP SLEEP. This state is the state of YOGA NIDRA (Yoga Nidra refers to a state of consciousness, not just the methods that lead you there). Nidra means sleep (not Dreaming), and Yoga means union, where all of those other thoughts and impressions have receded back into the unified field from which they arose.
Yoga Nidra – Conscious Deep Sleep
Companions to the higher goal: This is not yet the higher goal of sadhana (spiritual practices), which is to experience consciousness itself, independent of any grosser false identification, etc. (one might call this purusha, atman (soul), or the experience of the state of turiya, the fourth, but that’s a different thing, which is just beyond the level of conscious Deep Sleep, the goal of Yoga Nidra). However, Meditation and Yoga Nidra work together, as companion practices, in preparing one for such higher experience.
Yogic Conscious Deep Sleep by Swami Jnaneshvara Bharati at http://www.swamij.com/yoga-nidra.htm