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SB 3.11.22

tri-lokyā yuga-sāhasraṁ
 bahir ābrahmaṇo dinam
tāvaty eva niśā tāta
 yan nimīlati viśva-sṛk
Synonyms: 
tri-lokyāḥ — of the three worlds; yuga — the four yugas; sāhasram — one thousand; bahiḥ — outside of; ābrahmaṇaḥ — up to Brahmaloka; dinam — is a day; tāvatī — a similar (period); eva — certainly; niśā — is night; tāta — O dear one; yat — because; nimīlati — goes to sleep; viśva-sṛk — Brahmā.
Translation: 
Outside of the three planetary systems [Svarga, Martya and Pātāla], the four yugas multiplied by one thousand comprise one day on the planet of Brahmā. A similar period comprises a night of Brahmā, in which the creator of the universe goes to sleep.
Purport: 

When Brahmā goes to sleep in his nighttime, the three planetary systems below Brahmaloka are all submerged in the water of devastation. In his sleeping condition, Brahmā dreams about the Garbhodakaśāyī Viṣṇu and takes instruction from the Lord for the rehabilitation of the devastated area of space.