I have a large field of sunflowers in Pittsburgh Pa (40 deg latitiute)
every summer and I am puzzled as why my sunflowers do no follow the
sun. They pretty much always face East. I have other fields in
Switzerland  (45 deg latitue) and they do turn with the sun there.

I am using Greyseed sunflowers and they are usually pretty tal (5-8'
tall)

I have read somewhere that this si due to the fac that Pittsburgh
might not have enough difference in temperature during the day and
night.

Hello francois777-ga,

I am posting this as a comment since your plants in Switzerland do
turn with the sun.
  
The following is from the National Sunflower Association's website:

"When the plant is in the bud stage, it tends to track the movement of
the sun across the horizon. Once the flower opens into the radiance of
yellow petals, it faces east. No one knows why. However, it is likely
a defensive response. Facing south or west could result in
sun-scalding of seeds during very hot days."
http://www.sunflowernsa.com/pubinfo/faq/index.asp#11

--Bobbie7-ga