Yoga nidra was not known to people, just as kriya yoga was not known to people before Paramahamsa Yogananda, Lahiri and Mukteshwar. They discovered it and developed it. In the same way yoga nidra was developed by me.
Yoga nidra is taken from a tantric practice called anga nyasa. Anga means main limbs or organs of the body like the toes, knees, hips, the back, chest, shoulder blades and head. Nyasa means to place. In tantra, when you sit for pooja, the mind is placed on the various organs of the body along with the mantras - om netra netram, om shrotam shrotam. You touch each part and say a mantra.
The second stage is placing the mind on different parts of the hand, called kara nyasa. Kara means hand, fingers, wrist, etc. The third is varna nyasa, where the mind is placed on a colour; varna means colour. There are many nyasas in tantra and they are all done in siddhasana or padmasana. Through anga nyasa, varna nyasa, kara nyasa, the elements of the body are purified.
There are many types of purification in tantra. Bhuta shuddhi is purification of the elements of the body. Prana shuddhi is purification of the life force. When you are practicing tantric worship of yantra _ Sri Yantra, Sri Pooja, Devi Pooja, or Kali Pooja _ before the actual practice of pooja, prayer, and chanting, before the invocation and before the actual rites begin, you have to purify yourself. This process of purification is known as shuddhi.
The simple shuddhi is to take a bath and change your cloths, but this is not the only shuddhi. You have more in you. You have the nervous system and stomach, prana, nadis and elements. Aren't you going to purify them? Why confine purity to certain external factors only? Anga nyasa and kara nyasa are considered to be one step of this purification from which I have compiled the practice of yoga nidra.
When you practice yoga nidra you should not try to concentrate or practice awareness of the instructions intensely. The practice of yoga nidra has to be very casual. You should not be tense or try to be introverted and go deep. Rather you should try your best to maintain external awareness side by side with internal awareness.
The practice of yoga nidra is very difficult. People become tense, and because of tension they try to relax. Why should you try to relax? Relaxation happens, you don't have to make an effort. When you try to relax, that's not relaxation. Trying to relax creates tension. When relaxation happens naturally, it is relaxation.
When you are practicing yoga nidra, there are certain instructions _ right hand thumb, second finger, third finger, fourth finger, fifth finger. You just listen and think like that. If you do become introverted, you can murmur a little bit. The ingoing tendency of the mind must be arrested at a slightly deeper level than the conscious mind. You don't have to go very deep. Yoga nidra is not hypnotism and you do not forget what is happening.
When you practice yoga nidra, psychic experiences must be avoided. Psychic experiences happen mostly to people when the awareness is a little bit lower than the level of the coccyx. Of course, they can happen to madcaps anytime, but for most people, psychic experiences take place when the brainwaves change.
With normal brainwaves you cannot have psychic experiences. You have only thought experience and awareness, not psychic awareness. You don't feel heat and cold, angels and demons. You only feel it when the brainwaves change from the normal range.
When you are trying to practice yoga nidra, just lie down and tell your mind that you are going to take a rest. Let Swamiji say, "Right hand thumb, right hand finger, I don't care." Listen casually and when you feel that you are about to sleep, you just say, "No, I am not going to sleep." You must avoid sleep, introversion and the deep state of mind.
You must stay on the border line, almost on the fringe of consciousness, where you know that you are in this room, that the fans are working, the electricity is on and somebody is walking on the balcony. You should not go far, because if you go far, then yoga nidra is not yoga nidra, but becomes a part of meditation.
—Sivanandashram, Munger, 16 October 1982