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. For this reason, yoga nidra is often referred to as psychic sleep or deep relaxation with inner awareness.
In this threshold state between sleep and wakefulness, contact with the subconscious and unconscious dimensions occurs spontaneously. In Yoga Nidra, the state of relaxation is reached by turning inwards, away from outer experiences. If the consciousness can be separated from external awareness and from sleep, it becomes very powerful and can be applied in many ways, for example, to develop the memory, increase knowledge and creativity, or transform one's nature. (reference)
In the Raja yoga of Patanjali there is a state called pratyahara where the mind and mental awareness are dissociated from the sensory channels. Yoga Nidra is one aspect of pratyahara (sense withdrawal) which leads to the higher states of concentration and Samadhi. Yoga Nidra was devised by Sri Swami Satyananda Saraswati from traditional Tantric practices and has had a major impact on the world of yoga. Yoga Nidra is a pratyahara technique in which the distractions of the mind are contained and the mind is allowed to relax. This practice has had a profound transformative effect on practitioners. In Yoga Nidra, the consciousness is in a state between waking and sleep, but it is subject to neither.
Sleep is a natural, regularly occurring condition of generalized rest and relaxation of the mind and body, characterized by the absence of conscious thought, sensation or movement. Sleep is a natural form of pratyahara which occurs as our consciousness spontaneously detaches itself from the sensory and motor channels of experience. When the consciousness dissociates itself from both the sensory organs (jnanendriyas) and motor organs (karmendriyas), contact between the sensory/motor cortex of the brain and the external world is gradually lost. As this occurs, the consciousness is progressively withdrawn and redirected internally towards its source.
Yoga Nidra is actually a very simple practice, and you can learn it from a tape or record. Before starting, choose a quiet room and close the windows and doors. Turn off the television or radio, loosen your collar and tie, and switch on the tape recorder. Then lie down in corpse pose (shavasana) and listen to the instructions. Go on following the instructions mentally. Do not concentrate, do not control your breath, just listen to the instructions and follow them mentally. The most important thing in Yoga Nidra is to refrain from sleep. If you fall asleep, you lose the awareness that you are aiming for in the practice. The mind is then focused on external sounds. If all sensory impressions were forcibly excluded, then the mind would become restless and disturbed. Therefore, the mind is directed to think of external sounds, and to move from sound to sound with the attitude of a witness. After some time the mind loses interest in the external world and automatically becomes quiet.
Rotation of consciousness through the different parts of the body is not a practice of concentration, and it does not involve any physical movement, During the practice there are only three requirements to be fulfilled: (i) remain aware, (ii) listen to the voice, and (iii) move the mind very rapidly according to the instructions. When the instructor says `right hand thumb', repeat it mentally, think of the right hand thumb and move on. It is not necessary to be able to visualize the different body parts. You have only to become accustomed to following the same sequence, mentally repeating the names of the different body parts in the same way that the child learns to repeat the letters of the alphabet. You don't have to remember what part comes next. The whole process takes place in the subconscious mind.
The rotation of consciousness in Yoga Nidra proceeds in a definite sequence, beginning with the right thumb and ending with the little toe of the right foot; then the circuit from the left thumb to the little toe of the left foot. Subsequent circuits proceed from the heels to the back of the head, and from the head and individual facial features to the legs. After these rotations of consciousness have been completed, physical relaxation is then continued and completed by drawing attention to the breath. In this practice, one simply maintains awareness of the breath; there should be no attempt to force or change it. One may watch the breath in the nostrils, in the chest, or in the passage between the navel and the throat. Usually, greater relaxation is attained by simultaneously counting the breaths mentally. Awareness of the breath not only promotes relaxation and concentration, but also awakens higher energies and directs them to every cell of the body.
Next comes relaxation on the plane of feelings and emotions. Feelings that are intensely physical or emotional are recalled or awakened, experienced fully, then removed. Usually this is practised with pairs of opposite feelings, such as heat and cold, heaviness and lightness, pain and pleasure, joy and sorrow, love and hate. The pairing of feelings in Yoga Nidra harmonizes the opposite hemispheres of the brain and helps in balancing our basic drives and controlling functions that are normally unconscious. This practice also develops willpower on the emotional plane and brings about emotional relaxation by means of catharsis, as memories of profound feelings are relived.
The last stage of Yoga Nidra induces mental relaxation. In this part of the practice, the student visualizes the images named or described by the instructor. Since the images that are used often have universal significance and powerful associations, they bring the hidden contents of the deep unconscious into the conscious mind. The images used in this book include landscapes, oceans, mountains, temples, saints and flowers, stories, and descriptions of powerful psychic symbols such as the chakras, the lingam, the cross, or the golden flower.