The basic difference between the ordinary sleep and Nidra yoga is consciousness. Nidra yoga is conscious sleep. While practicing Nidra yoga you can be conscious and at the same time experience what you do at the time of sleeping. This actually allows us to scan thru all the aspects of our mind without making any physical efforts. In addition to this we also visualize the different parts of our body one after the other in a cyclic manner.
In the conscious state of our mind, when the two hemispheres of our nervous system are joined together, our reflexes work when ever we touch something very hot by mistake or when a mosquito bytes. The action is over even before we are able to think and understand what has happened. But this is said to the conscious state of mind because the moment we sense something our body responds.
If we go to sleep in a disturbed state of mind we tend to dream negative and continue to remain uncomfortable. Because of the discomfort through out the night, we are not fresh when we get up in the morning. There lies the difference between the ordinary sleep and Nidra yoga. In Nidra yoga since we consciously drive our thoughts we separate the connection between the two hemispheres of the nervous system. Separating the two hemisphere frees the mind from the physical problems of the body. By doing this our mind can go thru experiencing many different environment without actually creating those environment physically. We actually need to be prepared to experience these new things otherwise they can surprise us to the extent of frightening. Whether we try to meditate thru mantra or by concentrating on our breathe or otherwise these experiences can take us all the way away form what we are practicing.
According to the fifth chapter of the Bhagvad Gita that discusses about our nervous system, the brain of a main is like an inverted tree with its roots at the top and the branched and leaves below. Founded and designed by Swami Satyananda Saraswati, nidra yoga has been derived from tantric mode of worship.
The first state of meditation called Pratyahara is also the same in nidra yoga so the practice of nidra yoga in a sense is a way to prepare ourselves. Pratharya is a state of the nervous system when the mind is free from it. This state can not be experienced in the conscious state of mind. If we consider sleeping to be a slow process then the in between stage i.e. before falling asleep and after the state of consciousness is the stage of nidra yoga.