A reader sent me two more images of languid women for consideration.
Repose (Nonchaloire), John Singer Sargent, 1911, National Gallery of Art, Washington DC http://jssgallery.org/Paintings/Repose_(Nonchaloire).htm
Again, look at the hands which bring tension into the pose.
The Artist’s Wife and His Setter Dog, Thomas Eakins ca. 1884–89, Metropolitan Museum of Art
http://www.metmuseum.org/TOAH/hd/eapa/hob_23.139.htm
To me this is the most difficult position to paint. It’s easy to inadvertently make the figure rise in steps like a Ziggurat. That’s especially risky in this situation, where the gown stands so off so intensely from the background. Eakins twisted the torso slightly and further modified the blue shape with the shadows at the front of the skirt and the bodice, creating a lovely, flowing gown.