Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 22

Sanskrit:

त्रिलोक्या युगसाहस्रं बहिराब्रह्मणो दिनम् ।तावत्येव निशा तात यन्निमीलति विश्वसृक् ॥ २२ ॥

ITRANS:

tri-lokyā yuga-sāhasraṁbahir ābrahmaṇo dinamtāvaty eva niśā tātayan nimīlati viśva-sṛk

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.