The Yoga-nidra meditation is now used to get people to sleep which is great but the original intent was much more than catching up on sleep. During this session of Yoga nidra you will preferably lay on your back, Shavasana style and stay motionless in body and in mind while keeping you, the observer awake. The intent is to help you connect with your deepest self. The trick to benefit from Yoga-Nidra is to stay awake while the rest of you falls asleep. Your body, your mind fall asleep, you may be experiencing Alpha waves or relaxation then Theta waves or dream state and if you can maintain awareness finally Delta waves or deep sleep. What happens at this point I am not so sure because there is nothing, no activity but when you come back from it you have no idea how long you were there and you have that wonderful sense of being complete and regenerated. It's a special experience, as if you had reunited with a part of yourself you have not seen for a long time.