त्रिलोक्या युगसाहस्रं बहिराब्रह्मणो दिनम् ।
तावत्येव निशा तात यन्निमीलति विश्वसृक् ॥ २२ ॥
trilokyah-of the three worlds; yuga-the four yugas; sahasram-one thousand; bahih-outside of; abrahmanah-up to Brahmaloka; dinam-is a day; tavati-a similar (period); eva-certainly; nisa-is night; tata-O dear one; yat-because; nimilati-goes to sleep; visvasrk-Brahma.
Outside of the three planetary systems [Svarga, Martya and Patala], the four yugas multiplied by 1,000 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.
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.