Some patients may have obstructive breathing events in certain positions. Usually the worst position is lying on the back. Sewing a tennis ball in the back of the bed-clothes will help keep you off your back and on your side, which will help you breathe easier. Body positioning is monitored during the sleep study to help identify this problem.
In some cases, the tonsils may be larger and lying on the side may make obstructive events worse, and lying on the back may help here.