Learn several guided practices leading to the subtlety of Yoga Nidra.
Understand the subtler, deeper meaning of Yoga Nidra, not merely surface level relaxation.
Ordinary people do deep sleep inertly. Yogis experience deep sleep consciously.
Know your consciousness at all levels, waking, dreaming, AND deep sleep.
Yoga Nidra, conscious deep sleep, is the deepest level of awareness, just before samadhi, the highest level. Seek the highest.