SB 1.3 - The three forms of Viṣṇu
The puruṣa avatāra is manifested in three features – first the Kāraṇodakaśāyī Viṣṇu, who is a portion of Sankarṣaṇa, who creates aggregate material ingredients in the mahat-tattva, second the Garbhodakaśāyī Viṣṇu, who is a portion of Pradyumna, who enters in each and every universe, and third the Kṣīrodakaśāyī Viṣṇu, who is an expansion of Aniruddha, who is the Paramātmā of every material object.
Mahā-Viṣṇu or Kāraṇodakaśāyī Viṣṇu lies down on the ocean of kāraṇa, from where He glances over His material nature, and the mahat-tattva is at once created. Thus electrified by the power of the Lord, the material nature at once creates innumerable universes. Mahā-Viṣṇu is the inner soul of prakṛti. Material nature has no power to create without the power of the puruṣa. Because the Lord wished to lie down in meditation, the material energy created innumerable universes at once in the pores of His body, and in each of them the Lord lay down, and thus all the planets and different paraphernalia were created at once by the will of the Lord. This form, Garbhodakaśāyī Viṣṇu is the inner soul of the collective jīvas in each universe. Kṣīrodakaśāyī Viṣṇu is the inner soul of each jīva.
When Mahā-Viṣṇu developed a desire to lie down in a particular place, He lay down in the Kāraṇa ocean. He then glanced at prakṛti as soon as it came out with His breathing. By that glance He produced the mahat-tattva and other elements, and after creating the universes out of the elements, Garbhodakaśāyī Viṣṇu was informed to enter it and go to sleep. Entering into the universe, this Viṣṇu went to sleep for a second. He rejected the universe as a useless sleeping place, since it is material. So He leaves the universe after a second, which is the lifetime of Brahmā. Then again, He is made to sleep in a new universe for the lifetime of Brahmā, which is again a nimesa (less than a second) for Him.