Yoga Nidra in Hinduism is a kind of sleep induced by the technique of yoga. There is a 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.
Yoga Nidra is a method of exploring the different layers of the mind. According to certain schools of modern psychology, there are three dimensions of mind – the conscious, the subconscious and the unconscious. Mandukya Upanishad also classifies the mind into three layers – jagrat, swapna and sushupti. The yogic and psychological terminology can be related as follows:
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Jagrat – conscious mind, waking state, surface thoughts and perception of the outside world; sthula (gross dimension)
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Swapna – subconscious mind, dream state, individual memory and mind; sukshma subtle dimension.
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Sushupti – Unconscious mind, deep conscious sleep state, memory; karana (causal dimension).