In Drona Parva of Mahabharata, Krishna tells Arjuna about his four forms:
I have four forms, eternally engaged as I am in protecting the worlds. Dividing my own Self, I ordain the good of the worlds. One form of mine, staying on the earth, is engaged in the practice of ascetic austerities. Another beholdeth the good and the evil deeds in the world. My third form, coming into the world of men, is engaged in action. My fourth form lieth down in sleep for a thousand years. The form of mine which awaketh from sleep at the end of a thousand years, granteth, upon awakening, excellent boons to persons deserving of them.
It is generally believed that he sleeps after dissolving all mobile and immobile creatures into himself so how can Lord Vishnu give boon when no one else exists? As said by him, upon awakening he gives boon to deserving persons which means while in sleep, persons already exist which further means that he went to sleep when universe was running.
So why he sleeps in the middle of a running universe?