Yoga nidra means sleep with a state of inner awareness. There is a big difference between ordinary sleep and psychic sleep in yoga nidra. In sleep there is no awareness, whether internal or external; in yoga nidra, on the other hand, there is wakefulness to the internal environment. No difference, however, will be detected by an external observer.
The essence of yoga nidra is awareness. You should try to avoid sleep (unless you suffer from insomnia and are practising as a means to induce sleep). Before you begin, it is a good idea to mentally repeat to yourself: “I am going to relax myself completely but I am not going to sleep.” This resolution can be repeated at various times throughout the practice. Sleep is very difficult to control in yoga nidra because one becomes so relaxed. Even if you have slept for ten hours, you will easily drop into a state of slumber in yoga nidra. This resolve will help to maintain awareness.
Sleep is not yoga nidra. You have to maintain second to second awareness, keep alert and sharp, but without excessive tension. You must find the balance between too much effort and no effort. One leads to more tension and the other to sleep.
When you practise you will find that there will be fluctuation between periods of awareness and unawareness. Gradually as you master the practice and the mind becomes progressively calmed you will be able to maintain awareness. You will maintain inner awareness even though the external perceptions have been gradually dimmed. You will cross the threshold and experience spontaneous internal awareness, the purpose of yoga nidra. From this point you will be able to observe the subconscious and superconscious aspect of the mind. At this stage the techniques of yoga nidra, utilized to attain this state, can be discarded.
The method of maintaining awareness is to fix your attention on the sound of the instructor’s voice and the mechanics of the practice. This is the only way to avoid sleep.