The Upanishads are ancient spiritual texts, written in Sanskrit, by various authors, between 800 and 400 BC.
A man must perceive before he can speak the truth, so you need to understand perception first.
Prajapati said that you should try to discover the Self that is free from evils, old age, death, sorrow, hunger, thirst. The Self whose desires and intentions are real will have all desires fulfilled.
The gods and demons heard this, and separately each sent a representative ( Indra & Virocana) to find out how to obtain all one's desires.
They lived with Prajapati for 32 years, then asked how to discover the Self and obtain all desires. He said look at a reflection and see yourself. Virocana was content, but in a while, Indra realized that his body, even when well dressed up, would die. Prajapati said stay here another 32 years.
Then Indra asked again and was told that the one who goes about happily in a dream is the Self. At first he was satisfied, but then he realized that even in a dream bad things could be experienced.
So Indra went back and again was told to stay another 32 years.
After this, Indra asked again, and was told that when one is fast asleep, totally collected and serene and seeing no dreams, then one knows the Self. At first Indra was satisfied, but then again he had doubts, because in this dreamless state he was totally annihilated.
So Indra went back again and was told to stay another 5 years.
Then Prajapati said the Self has no body, it is immortal. The Self can see, smell, speak, hear, think but it has no body and is venerated by the gods. The Self perceives objects of desire found in the world of Brahman.
Question 4. Explanation of dream and dreamless sleep.
In sleep, the senses sleep, but the breath stays awake. The mind may re-experience what has gone before or may experience anew, and so has the potential to know the whole world and be the highest self (atman).