wikipedia: Heliotropism

“In the case of sunflowers, the uniform alignment of the flowers does result from heliotropism in an earlier development stage, the bud stage, before the appearance of flower heads. The apical bud of the plant will track the Sun during the day from east to west, and then will quickly move west to east overnight as a result of the plant’s circadian clock. The buds are heliotropic until the end of the bud stage, and finally face east.”

Imma checking!