Bhagavad Gita Chapter 8, Verse 18

अव्यक्ताद्व्यक्तय: सर्वा: प्रभवन्त्यहरागमे | रात्र्यागमे प्रलीयन्ते तत्रैवाव्यक्तसंज्ञके ||

avyaktād vyaktayaḥ sarvāḥ prabhavanti ahar-āgame rātry-āgame pralīyante tatraiva avyakta-saṁjñake

At the arrival of Brahma's day, all manifest beings come forth from the unmanifest; at the arrival of his night, they merge into that same unmanifest state.

Krishna describes the cycle of manifestation: 'Avyaktad vyaktayah sarvah prabhavanti aharagame'—all (sarvah) manifest beings (vyaktayah) come forth (prabhavanti) from the unmanifest (avyaktat) at the arrival (agame) of Brahma's day (ahar). 'Ratryagame pralayante tatraiva avyakta-samjnake'—at the arrival (agame) of his night (ratri) they merge (pralayante) into that same (tatra eva) unmanifest state (avyakta-samjnake). This verse describes the cosmic cycle: all manifest beings come from the unmanifest and return to it. Day represents manifestation—beings come forth. Night represents dissolution—beings merge back. This is the cycle of creation and destruction. But the unmanifest itself is eternal—it's the foundation that never changes. This helps you understand: everything manifest is temporary, but the unmanifest (Brahman) is eternal. This is what you're preparing to return to at death—not the temporary manifest, but the eternal unmanifest.