I shall talk about it on Monday. But anyway, unless you are constantly sleepy, you should allow it. As Jundo says, we don't reject any experience. The idea of keeping the spine erect or give an upward stretch is frequently used, as well as Keizan's advice in his Zazen Yojinki when he instructs :
"When the mind sinks into dullness, raise attention above your hairline or before your eyes. When the mind scatters into distraction, place attention at the tip of the nose or at the tanden. After this rest attention in the left palm. Sit for a long time and do not struggle to calm the mind and it will naturally be free of distraction."
The way I see it, is not to do those things. To accept what is. If it is too bad, just do kinhin. Walk. And come back to just sitting. The right sitting is doing itself. fixing this, adjusting that are very artificial, somehow too contrived. The basic principle is that the right thing does itself when we drop our wrong doing, our bad habits. As long as we interfere with our sitting, we do it from a deluded space. Sit. Give a direction, upward. Don't do it, allow it. Trust the practice and surrender to your true form. Let it sit you.
easy to say...