no matter who we are, we have to sleep. Many people ask me how to sleep everyday. My suggestion is that sleep on right side is the best and back is second and sleep on left is the last. Never sleep on your stomach. When you sleep on tummy, in general, we turn our neck to side. In the end, we make artificial distortions of neck bone, which leads nerve impingement (subluxation). Even when people sleep on sides, they raise their arms higher then their shoulders, which leads another tightness of their neck and shoulders.
If someone sleeps on side more than back, I recommend that the height of the neck has to be parallel to the bed people sleep on. And also, when people sleep on their back more than their sides, the neck has to be parallel to the bed. Some people do use soft beds. Some others do use hard beds. Therefore, you have to add the firmness of the bed to think about the height of pillow, too.