"Satyananda Saraswati (25 December 1923 – 5 December 2009), was a sannyasin, yoga teacher and guru in both his native India and the West. He was a student of Sivananda Saraswati, the founder of the Divine Life Society, and founded the Bihar School of Yoga in 1964. He wrote over 80 books, including Asana Pranayama Mudra Bandha.
Yoga Nidra, which is derived from the tantras, is a powerful technique in which you learn to relax consciously. In yoga nidra, sleep is not regarded as relaxation. People feel that they are relaxing when they collapse in an easy chair with a cup of coffee, a drink or a cigarette, and read a newspaper or switch on the television. But this will never suffice as a scientific definition of relaxation. These are only sensory diversions. True relaxation is actually an experience far beyond all this. For absolute relaxation you must remain aware. This is yoga nidra, the state of dynamic sleep. Yoga nidra is a systematic method of inducing complete physical, mental and emotional relaxation. The term yoga nidra is derived from two Sanskrit words, yoga meaning union or one-pointed awareness, and nidra which means sleep. During the practice of yoga nidra, one appears to be asleep, but the consciousness is functioning at a deeper level of awareness.