To be absolutely relaxed there has to be a complete awareness and it is this, because of awareness yoga nidra is known as complete sleep - the practice with a trace of awareness. Yoga nidra is a systematic practice, which induces physical, mental, and emotional relaxation. The word yoga nidra has come from two different Sanskrit words; first is 'yoga' which means union or one pointed awareness and second part is 'nidra' which means sleep. So literal meaning of yoga nidra is a sleep with a trace of awareness.
In this practice, the person may look like sleeping but the consciousness makes the mind to travel into the deeper levels of awareness. Because of this reason the practice is known as psychic sleep or deep relaxation with inner awareness. In the practice of yoga nidra, the person is at a threshold state of wakefulness or sleep, thereby it is very easy to come in contact with the subconscious and unconscious state of mind.
The practice of yoga nidra is done in shavasan; it is a pose of a dead person. The practice of yoga nidra comes from a tantrik practice called 'nyasa', which means 'to place' the mental awareness at a particular part of the body. This practice was designed by Swami Satyanand Saraswati, the founder of yoga.
Normally because of too much of distraction our mind doesn't experience one particular thing for a long period of time. This is not our fault; the mind keeps jumping from one object to next and it creates constantly subconscious and unconscious psychological stress. It is very easy to deal with the external stress but dealing with internal stress is difficult. But the practice of yoga nidra will help to deal with the internal stress, so the practice of yoga nidra is deeply rejuvenating and relaxing.