Brihadaranyaka Upanishad interprets and describes vividly waking, dreaming and deep sleep states experienced by the self/soul of a being. Every being has a self. This self is nothing but consciousness. Whatever is seen in the body--i.e. gross body, life-force and mind--is the manifestation of this self.
Self is eternally present. It always remains in a body, be it gross or subtle. When the gross body is destroyed at death, it remains in a subtle body. Subtle body is not material but imaginary. A single body is the imagination and manifestation of a single self whereas the world is the imagination of all selves. All these things can be understood by understanding waking, dreaming and deep sleep states as described below.
Waking state. In the waking state, a being of self sees its own gross body and the gross world outside. It enjoys the world through its mind and senses. It acts and receives the fruits of actions, which may be good or bad. As one is tired of actions and enjoyments, one goes to sleep and first enters into dreaming state.
Dreaming state. In this state, self remains in a subtle body. Through imagination, it creates its own illuminated subtle body as well as other illuminated worldly beings and objects. Here, self is self-illuminated. Self creates its own illuminated world and becomes its master. While the world of waking state is an imagination of all beings, dreaming world is an imagination of a single being. With the same mind and senses, a being enjoys its own dreaming world.
This state is very important from a spiritual point of view. In this state, a being experiences two worlds - one is the present world of its present birth and the other is the afterworld one shall experience after death. So a being sees its afterworld daily in its dreams. The afterworld is its own imagination and creation. According to its own action and imagination, the afterworld shall either be hell or heaven. In a dream, a being is engaged in actions and according to its actions, it receives either good or bad results (fruits). Then, a being enters into a deep sleep state.
Deep sleep state. In waking and dreaming states, self remains in dualism; for this, one sees other beings or objects. Mind and senses remain active. But as soon as self enters into deep sleep, body (both gross and subtle), mind and senses remain in complete rest. Self remains in its pure state--it remains alone. There is no dualism. There is none other than the self. For this, one sees nothing and hears nothing. There is no relation with the world. In this state, a father is not a father, a mother is not a mother, a son is not a son, a sinner is not a sinner, a king is not a king, a slave is not a slave and a patient is not a patient. Thus in this state, a being enjoys bliss. Beings are living for the bliss they get in deep sleep. Happiness from food, drink, sex, power or worship is momentary. But the happiness of being alone in deep sleep is complete. As a tired bird returns to its nest for rest in the evening, so a being comes to its pure state for enjoying complete bliss.
After enjoying bliss, the self returns to the dreaming state and to the waking state for worldly actions and enjoyments. In deep sleep, the mind is at rest or the mind remains in an unconscious state. However, if one can enter into deep sleep with a fully conscious mind, one shall get tremendous bliss and vigor. One shall realize the self. The way and means to enter into deep sleep with a conscious mind is yoga. It requires years of practice by controlling body, mind and senses. When one is able to enter into deep sleep with a conscious mind, it is called total concentration (Samadhi). When the mind is totally concentrated, one enters into a still higher state called Turiya. Here the self is pure consciousness. The aim of yoga and spiritualism is to reach this state right in this lifetime. When self is realized, everything is realized.
