Yoga Nidra, which is derived from the tantras, is a powerful technique in which you learn to relax consciously. In yoga nidra, sleep is t 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.
Swami Satyanada Saraswati is an authority on Yoga and more than anything else the popularity of his other books more than prove his prowess in his intense knowledge and the easy to understand language style. He is the Founder of Bihar School of Yoga, the only school of it's kind.