"Facing east at sunrise, the sunflower’s head follows the sun across the sky to face west at sunset. This heliotropic movement, called nutation, results from a bending of the stem toward the sunlight; there is asymmetric growth on the shaded side, causing the flowers to be in a position facing toward the sun. Growth is equalized during the night, the stem slowly straightens out and by dawn is facing east again. On a cloudy overcast day, the sunflower remains facing eastward, awaiting the next clear sunrise. The leaves of the sunflower are also heliotropic. If they are removed, the sunflower head would be unable to follow the sun.
When sunflower plants reach maturity, most flower heads permanently face east while their backs change color from green to yellow. "
My safflowers do the same thing, but there's enough tree shade toward the West that they quit turning around Noon-1pm .