Yoga nidra (Sanskrit: योग निद्रा) or yogic sleep is a state of consciousness between waking and sleeping, like the “going-to-sleep” stage, typically induced by a guided meditation. his state of consciousness is different from meditation, in which concentration on a single focus is required. In yoga nidra the practitioner remains in a state of light withdrawal of the 5 senses (pratyahara) with four senses internalized, that is, withdrawn, and only hearing still connects to any instructions given. The goals of both yogic paths, yoga nidra and meditation are the same, a state of meditative consciousness called samadhi.
It is among the deepest possible states of relaxation while still maintaining full consciousness. In lucid dreaming, one is only, or mainly, cognizant of the dream environment, and has little or no awareness of one’s actual environment.
Yoga nidra results in conscious awareness of the deep sleep state, which is called prajna in the Mandukya Upanishad.
Yoga nidra is a state in which the body is completely relaxed, and the practitioner becomes systematically and increasingly aware of the inner world by following a set of verbal instructions. The regular practice of yoga relaxation has been found to reduce tension and anxiety. The autonomic symptoms of high anxiety such as headache, giddiness, chest pain, palpitations, sweating and abdominal pain respond well. It has been used to help soldiers from war cope with posttraumatic stress disorder (PTSD)
source : https://en.wikipedia.org/wiki/Yoga_nidra