BG 8.17-19
The people who know the day of Brahmâ, a thousand ages in duration, and the night a thousand ages in ending, they know day and night. From the unmanifested all the manifested stream forth at the coming of day; at the coming of night they dissolve, even in That called the unmanifested. This multitude of beings, going forth repeatedly, is dissolved at the coming of night: by ordination, O Pârtha, it streams forth at the coming of day.