Yoga nidra is not what you think. It was created by Swami Satyananda Saraswati in the 1960s as a result of his being able to remember chants that he did not recall being exposed to. It turns out that he was exposed to them while he slept and young boys chanted them at a monastery in India where he was stationed. Ancient yogis, indeed Patanjali in the Yoga Sutras (1:38) make mention of reaching samadhi through contact with sleep and dreams. This is one piece of yoga nidra, which is often referred to as yogic sleep.