This article examines Yoga Nidra — the ancient Indian practice of systematic guided relaxation that induces the hypnagogic state between waking and sleep — through current neuroscience, clinical research, and its traditional framework in the Mandukya Upanishad and Tantric tradition. The neuroscientific evidence is examined: Yoga Nidra practice produces delta and theta brainwave states comparable to deep sleep while maintaining conscious awareness, produces measurable reductions in cortisol and sympathetic nervous system activation, and has been shown to improve sleep architecture, reduce anxiety and PTSD symptoms, and enhance HRV. The mechanism is examined through the lens of the parasympathetic nervous system activation, the default mode network, and the specific hypnagogic transition state that the practice systematically inhabits. Five specific evidence-based benefits are documented: stress cortisol reduction, sleep quality improvement, anxiety and PTSD relief, creative and intuitive capacity enhancement, and the cultivation of the witness consciousness that the Mandukya Upanishad identifies as the fourth state — Turiya. Stanford neuroscientist Andrew Huberman's NSDR (Non-Sleep Deep Rest) protocol is contextualised as a secularised adaptation of the same practice.