When you lay down, the pressure on your back is removed. If you feel laying down is a problem, use a pillow underneath your head. It has the same waves as the sun(without the harmfull ones)and is not only good but as i say, Vital!
You must be selling this standing up saunas that are getting a bit outta date. the best models and where you feel more comfortable are the ones you lay down.
Whats the next think? Can you not lay down to get a sun tan?