Origin of Yoga Nidra
The meditative sound of “aum” is composed of three letters—a, u and m. The “a” is the state of wakefulness, and external awareness through the mind and sense organs. The “u” represents the dream state, composed of internal experiences. The “m” represents the Yoga nidra state of deep, lucid sleep, being the third of four levels of consciousness. (The fourth level—absolute emptiness—is that of no sound with neither inward nor outward consciousness. This fourth level lies beneath the other three states.)