Yoga nidra
Yoga nidra (, ) or yogic sleep in modern usage is a state of consciousness between waking and sleeping, typically induced by a guided meditation. A state called yoga nidra is mentioned in the Upanishads and the Mahabharata, while a goddess named Yoganidrā appears in the Devīmāhātmya. Yoga nidra is linked to meditation in Shaiva and Buddhist tantras, while some medieval hatha yoga texts use "yoganidra" as a synonym for the deep meditative state of samadhi. These texts however offer no precedent for the modern technique of guided meditation. That derives from 19th and 20th century Western "proprioceptive relaxation" as described by practitioners such as Annie Payson Call and Edmund Jacobson. The modern form of the technique, pioneered by Dennis Boyes in 1973, made widely known by Satyananda Saraswati in 1976, and then by Swami Rama, Richard Miller, and others has spread worldwide. It is applied by the US Army to assist soldiers to recover from post-traumatic stress disorder. There is limited scientific evidence that the technique helps relieve stress. Historical usage Ancient times The Hindu epic Mahabharata, completed by the 3rd century CE, mentions a state called "yoganidra", and associates it with Lord Vishnu: The Devīmāhātmya, written around the 6th century CE, mentions a goddess whose name is Yoganidrā. The god Brahma asks Yoganidrā to wake up Vishnu to go and fight the Asuras or demigods named Madhu and Kaitabha. These early mentions do not define any yoga technique or practice, but describe the god Vishnu's transcendental sleep in between the Yugas, the cycles of the universe, and the manifestation of the goddess as sleep itself. Medieval practices Yoganidra is first linked to meditation in Shaiva and Buddhist tantras. In the Shaiva text Ciñcinīmatasārasamuccaya (7.164), yoganidra is called "peace beyond words"; in the Mahāmāyātantra (2.19ab) it is named as a state in which perfected Buddhas may access secret knowledge. In the 11th or 12th century, yoganidra is first used in Hatha yoga and Raja yoga texts as a synonym for samadhi, a deep state of meditative consciousness where the yogi no longer thinks, moves, or breathes. The Amanaska (2.64) asserts that "Just as someone who has suddenly arisen from sleep becomes aware of sense objects, so the yogin wakes up from that [world of sense objects] at the end of his yogic sleep." By the 14th century, the Yogatārāvalī (24–26) gives a more detailed description, stating that yoganidra "removes all thought of the world of multiplicity" in the advanced yogi who has completely uprooted his "network of Karma". He then enters the "fourth state", namely turiya or samadhi, beyond the usual states of waking, dreaming, and deep sleep, "that special thoughtless sleep, which consists of [just] consciousness." The 15th century Haṭha Yoga Pradīpikā goes further, stating (4.49) that "One should practice Khecarī Mudrā until one is asleep in yoga. For one who has achieved Yoganidrā, death never occurs." Khecarī Mudrā is the Hatha yoga practice of folding the tongue back so that it reaches inside the nasal cavity, where