Why do my 2' sunflower stalks droop all day and only stand up at night? They get watered twice a day.?

They grew from seeds and were planted when about 2' tall. They droop very badly all day and stand up straight over night. I water them in the morning and again at night. I thought they liked the sun but they look almost dead all day, then in the morning they are standing nice and tall. This is every day since they were planted about 3 days ago.