When Lord Vishnu sleeps?

It is believed that before the beginning of each rainy season, Lord Vishnu sleeps for four months. As a result, the annual pralaya takes place. It is believed that Lord Vishnu seeks a break as he is tired and sleeps after completing his work. This pralaya is the time when the world gets a new life. Lord Vishnu falls asleep on the Kshir Sagar or the ocean of milk and rests on the Shesha Naga. This day is called the Shayani Ekadashi. He later wakes up on the eleventh of Kartik month called Prabodhini Ekadashi which corresponds with the rainy season in India.