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Yoga nidra or "yogi sleep" is a sleep-like state which yogis report to experience during their meditations.
The practice of yoga relaxation[clarification needed] has been found to reduce tension and anxiety. The autonomic symptoms of high anxiety such as headache, giddiness, chest pain, palpitations, sweating, abdominal pain respond well. It has been used to help soldiers from war cope with PTSD.
History and background 
Yoga nidra was experienced by Swami Satyananda Saraswati when he was living with his guru Swami Sivananda in Rishikesh. He began studying the tantric scriptures and, after practice, constructed a system of relaxation, which he began popularizing in the mid 20th century. He explained yoga nidra as a state of mind between wakefulness and sleep that opened deep phases of the mind, suggesting a connection with the ancient tantric practice called nyasa, whereby Sanskrit mantras are mentally placed within specific body parts, while meditating on each part (of the bodymind). The form of practice taught by Swami Satyananda includes eight stages (Internalisation, Sankalpa, Rotation of Consciousness, Breath Awareness, Manifestation of Opposites, Creative Visualization, Sankalpa and Externalisation).
Teachers such as Osho and Anandmurti Gurumaa define yoga nidra as a state of conscious deep sleep. One appears to be sleeping but the unconscious mind is functioning at a deeper level: it is sleep with a trace of deep awareness. In normal sleep we lose track of our self but in yoga nidra, while consciousness of the world is dim and relaxation is deep, there remains an inward lucidity and experiences may be absorbed to be recalled later. Since yoga nidra involves an aimless and effortless relaxation it is often held to be best practised with an experienced yoga teacher who verbally delivers instructions.
Anandmurti Gurumaa taught two techniques based on creative visualization. Yoga nidra as Yoga of Clear Light is proposed as a spiritual path (sadhana) in its own right, held to prepare and refine a seeker (sadhaka) spiritually, emotionally, mentally and physically for consciousness and awareness. The yogi may work through the consequences of deeds (karma), cleansing the store consciousness and purifying the unconscious mind. The state may lead to realisation (samādhi) and being-awareness-bliss (satchitananda). The yogi is held to be in communion with the divine. A tantrika engaged in this sadhana may become aware of past or future lives (refer bhumi) or experience the astral planes.
Scientific evaluation 
Experimental evidence of the existence of a fourth state of unified, transcendental consciousness, which lies in the yoga nidra state at the transition between sensory and sleep consciousness, was first recorded at the Menninger Foundation in Kansas, USA in 1971. Under the direction of Dr. Elmer Green, researchers used an electroencephalograph to record the brainwave activity of an Indian yogi, Swami Rama, while he progressively relaxed his entire physical, mental and emotional structure through the practice of yoga nidra.
What they recorded was a revelation to the scientific community. The swami demonstrated the capacity to enter the various states of consciousness at will, as evidenced by remarkable changes in the electrical activity of his brain. Upon relaxing himself in the laboratory, he first entered the yoga nidra state, producing 70% alpha wave discharge for a predetermined 5 minute period, simply by imagining an empty blue sky with occasional drifting clouds.
Next, Swami Rama entered a state of dreaming sleep which was accompanied by slower theta waves for 75% of the subsequent 5 minute test period. This state, which he later described as being 'noisy and unpleasant', was attained by 'stilling the conscious mind and bringing forth the subconscious'. In this state he had the internal experience of desires, ambitions, memories and past images in archetypal form rising sequentially from the subconscious and unconscious with a rush, each archetype occupying his whole awareness.
Finally, the swami entered the state of (usually unconscious) deep sleep, as verified by the emergence of the characteristic pattern of slow rhythm delta waves. However, he remained perfectly aware throughout the entire experimental period. He later recalled the various events which had occurred in the laboratory during the experiment, including all the questions that one of the scientists had asked him during the period of deep delta wave sleep, while his body lay snoring quietly.
Such remarkable mastery over the fluctuating patterns of consciousness had not previously been demonstrated under strict laboratory conditions. The capacity to remain consciously aware while producing delta waves and experiencing deep sleep is one of the indications of the superconscious state (turiya). This is the ultimate state of yoga nidra in which the conventional barriers between waking, dreaming and deep sleep are lifted, revealing the simultaneous operation of the conscious, subconscious and unconscious mind. The result is a single, enlightened state of consciousness and a perfectly integrated and relaxed personality.
Dr. Kamakhya Kumar in 2006 awarded by Ph. D. degree by Dr. A. P J Abdul Kalam (President of India) for his work "Psycho-physiological Changes as Related to Yoga Nidra". He observed six months effects of yoga nidra on some Physiological, hematological and some Psychological parameters on the practitioners and he found a significant change on above mentioned parameters. One of the research published entitled "A study on the impact on stress and anxiety through yoga nidra; Indian Journal of Traditional Knowledge, Vol. 7 No 3".(Published through NISCAIR)
Indian clinical psychologist Sachin Kumar Dwivedi (2009) found in his research that yoga nidra decreases levels of anxiety. Dwivedi, S., Awasthi, S.& Pandey,B.B.(2011) found in "Yoga Nidra increased the α-eeg on α-eeg biofeedback." That is open scrate [secret?] that Yoga Nidra is a type of deep meditation. Nikhra,M & Dwivedi,S.K.(2010) found in a study "Yoga Nidra Reduces the Level of Stress."
See also 
- Bihar School of Yoga
- Bihar Yoga Bharati
- Dream yoga
- Mandukya Upanishad
- Ösel (yoga)
- Richard Miller (psychologist)
- Swami Niranjanananda
- Paramahamsa Satyananda
- Eileen Rivers, Washington Post Tuesday, May 6, 2008; Page HE01
- Rama, Swami. Mandukya Upanishad: Enlightenment Without God. ISBN 0-89389-084-7.
- Saraswati, Swami Satyananda (1974). Tantra-yoga panorama. International Yoga Fellowship Movement. p. 25. Retrieved 8 March 2011.
- Osho, The Secret of Secrets, Vol 1, Chapter 12. Rajneesh Foundation
- Green, E.E., Biofeedback for mind/body self-regulation, healing and creativity, in Academy of parapsychology and medicine (1972). The varieties of Healing Experience: exploring psychic phenomena in healing; transcript of the interdisciplinary symposium, Los Altos - Calif., October 30, 1971. Retrieved 13 April 2011.
- ANAND, B; CHHINA, G, SINGH, B (1 June 1961). "Some aspects of electroencephalographic studies in Yogis☆". Electroencephalography and Clinical Neurophysiology 13 (3): 452–456. doi:10.1016/0013-4694(61)90015-3. Retrieved 7 March 2011.
- Boyd, D. (1995). Swami: Encounters with modern mystics. Honesdale, PA: Himalayan Publishers.
- Brown, K.W., Ryan, R.M., & Creswell, J.D. (in press). Mindfulness: Theoretical foundations and evidence for its salutary effects.
- Kasamatsu, Akira; Hirai, Tomio (1 December 1966). "AN ELECTROENCEPHALOGRAPHIC STUDY ON THE ZEN MEDITATION (ZAZEN)". Psychiatry and Clinical Neurosciences 20 (4): 315–336. doi:10.1111/j.1440-1819.1966.tb02646.x. PMID 6013341.
- Dwivedi, S.K.(2009). The effect of Yoga nidra on Anxiety. Shodh Dhara, 12(2), 80-84
- Dwivedi, S.K., Awasthi.S. & Pandey,B.B.(2011). The efficacy of Yoga Nidra on α-eeg. Paper Presented at Seminar on Indigenous Techniques in Psychotherapy (26-03-2011). Best Paper Presentation Awarded
- Kumar, Kamakhya (2004) Yoga nidra and its impact on student’s well being; Yoga Mimamsha, Kaivalyadhama, Lonavla; Vol.36 No.1
- Kumar, Kamakhya (2005) Effect of Yoga nidra on hypertension and other psychological co-relates; Yoga the Science; Yoga Publications, Hubli, Karnataka; Volume 3, Issue 7.
- Kumar, Kamakhya (2006) A study of the improvement of Physical and Mental Health through Yoga nidra; Dev Sanskriti Journal, Vol. 4 Year
- Kumar, Kamakhya (2007) The Healing Sleep; Yoga Magazine Mind Body Spirit, York Street London, Issue 50 March.
- Kumar, Kamakhya (2008) A study on the impact on stress and anxiety through Yoga nidra; Indian Journal of Traditional Knowledge, Vol. 7 No 3.
- Kumar, Kamakhya (2010)Psychological Changes as related to Yoga Nidra; International Journal of Psychology: A Biopsychosocial Approach 2010 (6) pp 129–137
- Madhavananda (Swami.); N. S. Mital (1998). Lila-Amrit: the divine life of Sri Mahaprabhuji. International Sri Deep Madhavananda Ashram Fellowhip. Retrieved 7 March 2011.
- Nikhra.M. & Dwivedi,S.K.(2010). A Study of the Effect of Meditation on Stress. Indian Journal of Psychology and Mental Health,4(6),78-81
- Resting in Stillness: Integrative Restoration - iRest Yoga Nidra by Richard Miller PhD; first published 2008 by Anahata Press, ISBN 978-1-893099-09-8
- Saraswati, Swami Satyananda (1 August 2003). Yoga Nidra. Yoga Publications Trust. ISBN 978-81-85787-12-1. Retrieved 7 March 2011.
- Saraswati, Swami Yogeshwranand (1972). Science of soul: a practical exposition of ancient method of visualisation of soul. Yoga Niketan Trust. Retrieved 7 March 2011.
- Washington Post Article on study of Post Traumatic Stress Disorder using Yoga Nidra
- Science Direct Study of dopamine response during Yoga Nidra
- Human Brain Mapping Study using PET scans during Yoga Nidra
- Pictures of the brain's activity during Yoga Nidra Bindu Magazine article on Yoga Nidra research from The State University Hospital in Copenhagen