Life drawing is one of my favorite exercises. For one, a person is not just drawing one image... A person is melding two (due to binocular vision). This accounts in part for the subtle difference seen in drawings made from life and those made from photographs.
Our eyes, of course, take in two different images of the three-dimensional model before us. It demands a lot of energy, not to mention there is the pressure of the clock. You can't tuck the model into a notebook and save the drawing for later. And they aren't going to sit there forever, especially in a pose like this:
I can see in the mark making of the next picture that I was beginning to flag and get tired by the end of the set. But here is the last picture anyway, because there is something in it that I like.
Something about it reminded me of this old photocopied page from a book on Odilon Redon, which is taped to my wall: