In yogic philosophy, a sankalpa is a solemn vow, made in the heart and forged by the will. A yogi sets a sankalpa to focus the mind and heart on a particular goal. Like much of Hindu philosophy, the idea of sankalpa is complex and layered, but the Western practice of conscious intention-setting that has gained popularity in the past few decades could be considered a simplified conception of sankalpa, with the caveat that a sankalpa is meditative and process-oriented. When you set a sankalpa, the effort you make towards your goal is as important as achieving it.
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.