Humans spend about a third of their lives sleeping. Unconsciously. Just like breathing, sleep is believed to be especial for our survival, as means of relaxation and rejuvenation. We all practice it every day.
Yogis of the past developed a practice, derived from tantras, which allowed them to sleep continuously and direct their awareness towards achieving deeper and broader relaxation than in a state of sleep.
Swami Satyananda, in his book “Yoga Nidra” (ISBN: 8185787123) says:
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.
In the world and time of endless distractions, sensory overload, societal pressures and dissipated awareness sleep is just not enough. Regular practice of yoga nidra is essential for survival. Survival of sane mind and clear consciousness.
Yoga nidra is believed to be related on tantric rituals called “nyasa”:
“Nyasa means placing and refers to a large component of tantrik ritualism in which the practitioner touches various parts of the body at the same time pronouncing a mantra and visualising a devata or a bija (root) mantra. Nyasa is supposed to "divinise" the body of the worshipper.”
"Nyasa -- divinity in the body". Shiva Shakti Mandalam
More specifically - anga nyasa, varna nyasa and kara nyasa practices:
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.
Yoga Nidra / YogaMag / —Sivanandashram, Munger, 16 October 1982 (http://www.yogamag.net/archives/2011/invdc11/yn.shtml)
Looking into the original practices it becomes clear that yoga nidra is a process of purification of the body and mind, achieved through systematic practice of “dynamic” sleep with one-pointed awareness.
Important work in bringing Yoga Nidra to present day was shouldered by Swami Satyananda, who explored the old complicated techniques, re-shaped them, defined specific stages, and co\ame up with a sleek elegant system suitable for modern realities
Human mind has a layered structure, where the second layer after the conscious one is subconscious holding recent samskaras (mental impressions) to transit them to the deepest - unconscious layer (where samskaras are imprinted and processed to be utilized in consequent lifetimes).
Natural human sleep is marked by mind shifting to, firstly, unconscious, then subconscious states which are characterised by dreaming and next deep sleeping/non-dreaming attributes respectively.
Yoga nidra system is based on a deep relaxation technique, that employs an involuntary release of tension in sensory-motor cortex - regions of of the cerebral cortex. It unlocks the unconscious mind without going into a usual dreaming state of sleep (see Pic.1).
In yoga nidra awareness of the mind is drawn to certain areas in the body providing relaxation gradually, deepening pratyahara (sensory withdrawal).
Yoga nidra practice consists of 8 stages, 4 of which are essential:
1. Settling. Essential. Prepares the body to release physical tensions, draws attention to the senses to let the unsettled mind pacify and start the act of internalisation. This is the first level of pratyahara.
2. Sankalpa. Initial affirmation to start the process of transformation of practitioner’s personality.
3. Body Rotation. Essential. Complete physical relaxation comes with rotation of body parts in Motor Homunculus. All motor and sensual functions in the body are temporally suspended.
4. Breath Awareness. Essential. The 2nd level of pratyahara is achieved by taking attention to the breath. Internalisation stage.
5. Opposites. Emotional relaxation encouraged due to homeostatic balance created by stimulating hypothalamus (feelings) and limbic system (emotions). By the next stage pratyahara is complete, letting the unconscious mind to manifest. The 6th stage relaxes the mind making it free for concentration (dharana) and then, probably, to dhyana (pure meditation).
6. Visualisations. Mental relaxation comes with arising self-awareness by shifting attention along images in the instruction. At this stage transformation messages can be brought into play because mind tip is entirely located in the unconscious and is one-pointed.
7. Sankalpa. Repeated exactly as in the stage 2 into the unconscious mind to complete the transformation message and reaffirm intention.
8. Externalisation. Essential. Slow externalisation by attracting the awareness to the breath, body and surroundings.
Benefits of yoga nidra include mind purification, releasing of emotional and thought patterns, cleaning up accumulated karma, curing insomnia, addictions and stimulating willpower.
Yoga nidra is a cheat-trick all studying students and sleep-deprived parents should adopt, as means of achieving deep relaxation in short period of time.