The Mahabharata Translated by Vivek Debroy, Vana Parva chapter 194
When the world became one terrible ocean and all mobile and immobile objects and all beings had been destroyed, the illustrious Vishnu, the eternal and undecaying being, the source of everything in the world, slept alone on the ocean. He was stretched out on the great serpent Shesha, whose energy is limitless. O illustrious one! The immensely fortunate Hari, the undecaying one and the creator of the world, slept in the coils of that great serpent, which encircled the expanse of the earth.
During the night of brahmA, Bhagavan Vishnu goes to sleep. Nothing else happens.