At the approach of (Brahma’s) day, all manifestations proceed from the unmanifest state, and at the approach of the night they merge into that same state, called the unmanifest. (Bhagavad Gita 8.18)
Verses 17 and 18 of chapter 18 of Bhagavad Gita signify the evolution and involution of cosmic Energy, represented by Brahma’s day and night. As we go to sleep and become unconscious of the external world, so does Brahma also go to sleep. Then all becomes chaos and the world comes to an end. But when Brahma wakes up, evolution begins again. The universe was not annihilated during Brahma’s sleep, but in the process of involution everything goes back to its finer state. It becomes finer and finer until it is no more perceivable. But everything remains in the germ state, to come out again in the process of evolution. This process of evolution and involution, according to some, takes place with the entire Cosmos, while others hold that it refers to one solar system at a time, each solar system getting its turn.
That everything in nature works according to law and not in a haphazard way as some believe, and that everyone reaps the fruit of his own sowing, no more and no less.