Bhagavad Gita 8.18

8.18 From the Unmanifested all the manifested (worlds) proceed at the coming of the 'day'; at the coming of the 'night' they dissolve verily into ï1thatï1 alone which is called the Unmanifested.

At the beginning of Brahmā’s day, all living entities become manifest from the unmanifest state, and thereafter, when the night falls, they are merged into the unmanifest again.