I start sleeping on my side with my arm under the pillow and up towards the head of the bed. During the night my hubby says I sleep mostly on my back with my arms up. I have tried many things but not sure about the tennis balls as I have no way of securing them as I wear singlet top to bed(summer here)I used the sleep on my tummy as a kid but haven't slept this way for years. I will try google to see what I can find to stop sleeping this way. Thanks for your suggestion Jo. It may work putting tennis balls under the sheet in strategic positions