are no images in the mind. If there are images, that is the
Waking or the Dreaming state, not the Deep Sleep state.
Yoga Nidra relates to conscious Deep Sleep.
("Nidra" means "Deep Sleep" not "Dreaming")
Dreaming and Not-Dreaming are NOT the same thing.
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. 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 CD on Amazon.com
Yoga Nidra Article on SwamiJ.com