I don't think there is any rule-of-thumb as to the 'best' position when using a cpap. I have found however, that if the person sleep on their side with a pillow at their neck/head and one for between the slightly bent knees and ankles, is the most restful and easy breathing position. Always try to keep the pillow away from their face to ensure the mask not become un-positioned. To roll in the opposite direction half way through the night will assure a healthy and restful sleep. ~Sooz~
