Why do sunflower flowers face towards the sun?

According to research conducted in the year 2016, sunflower flowers have a biological clock. Which is known as helio tropism. Due to this process the sunflower flowers face towards the direction of the sun. Flowers rest during the night. As soon as the flowers become active in the morning, they become in the direction of the sun and become in the east direction and in the evening the direction of the flowers becomes towards the west.