Prashna Upanishad 4

Then Sauryâyanin Gârgya asked: "Sir, What are they that sleep in this man, and what are they that are awake in him? What power (deva) is it that sees dreams? Whose is the happiness? On what do all these depend?"

He replied: "O Gârgya, As all the rays of the sun, when it sets, are gathered up in that disc of light, and as they, when the sun rises again and again, come forth, so is all this (all the senses) gathered up in the highest faculty (deva) the mind. Therefore at that time that man does not hear, see, smell, taste, touch, he does not speak, he does not take, does not enjoy, does not evacuate, does not move about. He sleeps, that is what people say.

The fires of the prânas are, as it were, awake in that town (the body). The Apâna is the Gârhapatya fire, the Vyâna the Anvâhâryapakana fire; and because it is taken out of the Gârhapatya fire, which is fire for taking out, therefore the Prâna is the Âhavanlya fire.

Because it carries equally these two oblations, the out-breathing and the in-breathing, the Samâna is he (the Hotri priest). The mind is the sacrificer, the Udâna is the reward of the sacrifice, and it leads the sacrificer every day (in deep sleep) to Brahman.

There that god (the mind) enjoys in sleep greatness. What has been seen, he sees again; what has been heard, he hears again; what has been enjoyed in different countries and quarters, he enjoys again; what has been seen and not seen, heard and not heard, enjoyed and not enjoyed, he sees it all; he, being all, sees.

And when he is overpowered by light, then that god sees no dreams, and at that time that happiness arises in his body.

And, O friend, as birds go to a tree to roost, thus all this rests in the Highest Âtman,—

The earth and its subtile elements, the water and its subtile elements, the light and its subtile elements, the air and its subtile elements, the ether and its subtile elements; the eye and what can be seen, the ear and what can be heard, the nose and what can be smelled, the taste and what can be tasted, the skin and what can be touched, the voice and what can be spoken, the hands and what can be grasped, the feet and what can be walked, the mind and what can be perceived, intellect (buddhi) and what can be conceived, personality and what can be personified, thought and what can be thought, light and what can be lighted up, the Prâna and what is to be supported by it.

For he it is who sees, hears, smells, tastes, perceives, conceives, acts, he whose essence is knowledge, the person, and he dwells in the highest, indestructible Self,—

He who knows that indestructible being, obtains (what is) the highest and indestructible, he without a shadow, without a body, without colour, bright,— yes, O friend, he who knows it, becomes all-knowing, becomes all. On this there is this Sloka:

He, O friend, who knows that indestructible being wherein the true knower, the vital spirits (prânas), together with all the powers (deva), and the elements rest, he, being all-knowing, has penetrated all."