I tried that last weekend, setting up in the shade of a big elm tree, and blanked.
Last year I created a bivvy spot right under the willows and again, the fish seemed to avoid the area.
On both occasions the fish caught were in the brightest and sunniest spots.
I would say that I should have moved, but with Sky I refused as I would much rather she was comfortable as possible, and unless I tell her will lay in the sun all day and overheat.