SB 3.11.22
tri-lokya yuga-sahasram
bahir abrahmano dinam
tavaty eva nisa tata
yan nimilati visva-srk
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Outside of the three planetary systems [Svarga, Martya and Patala], the four yugas multiplied by one thousand comprise one day on the planet of Brahma. A similar period comprises a night of Brahma, in which the creator of the universe goes to sleep.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When Brahma goes to sleep in his nighttime, the three planetary systems below Brahmaloka are all submerged in the water of devastation. In his sleeping condition, Brahma dreams about the Garbhodakasayi Visnu and takes instruction from the Lord for the rehabilitation of the devastated area of space.