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 (See Karma article).
Yoga Nidra has been known for thousands of years by the sages and yogis. Of the three states of consciousness of Waking, Dreaming and Deep Sleep, as expounded in the Upanishads, particularly the Mandukya Upanishad, Yoga Nidra refers to the conscious awareness of the Deep Sleep state, referred to as prajna in Mandukya Upanishad. This is the third of the four levels of consciousness of AUM mantra, relating to the state represented by the M of AUM. The four states are Waking, Dreaming, sleep, and turiya, the fourth state. The state of Yoga Nidra, conscious Deep Sleep, is beyond or subtler than the imagery and mental process of the Waking and Dreaming states.
In Yoga Nidra, you leave the Waking state,
Excerpt from swamij.com Article Yoga Nidra