In the texts of Vedanta there are several discussions on the nature of sleep. It is said that sleep is just one step short of the supreme ananda: the bliss of divinity. If it is conscious it is ananda. If it is unconscious, it is just a blurring of awareness. A forced resting of the mind.
Our journey through the disturbance of noise through life ends again when the noise making instruments of the brain begin to shut down and one enters that total silence which the noisy ones among us fear. Only the noisy ones among us fear that. Others enter that silence, which the majority have named as death. From silence we begin and into silence we merge and re-enter, but we never quite forget our origin in silence. Every night we tire of the noise of the day and we enter the silence of sleep involuntarily. The only difference is that the yogi sleeps consciously, but the common person sleeps unconsciously.
Yoga-nidra (Yoga Sleep) is to be defined as the experience of a state of Conscious Sleep in which the sadhaka is showing all the symptoms of deep sleep, producing delta brain waves, and is at the same time fully conscious of the events in his surroundings. However, a lower state of yoga nidra begins with theta brain waves during which volitionally directed intuitive creativity begins.
The first preparatory level of yoga-nidra is a state of deep relaxation. Very commonly these days the term yoga-nidra is used for the processes/exercises preparatory to yoga-nidra. These are often also being taught by many teachers in a fragmentary manner. There are many different such preparations in complete sets, each exercise fitting at a certain place in the sequence in the set.
These relaxation and subtle body preparatory exercises lead to the true yoga-nidra, entry into the cognition of negation in a cave in the heart centre as stated in the Yoga-sutra 1.10.
The final stage of yoga-nidra is the kind of sleep that yogis sleep even up to three and half hours. Here the mind simultaneously remains at two levels: one layer of the mind in sleep in the ordinary sense of the word, and a deeper layer of the mind remaining in conscious a-japa japa and meditation which, here, is awareness of the person-wide awareness of kundalini. This may require an advanced teacher initially leading the aspirant into this depth.