-has to support your neck and head
-not so soft nor hard pillow nor mattress
-when you lay down on a side on a mattress with a pillow, your nasal septum (nose center) has to be parallel to the mattress.
-your head and neck can not tilt nor wedged, rotated (snoring position or teeth grinding).  has to be straighgt.
-do not sleep on tummy
-if you sleep on your back, pillow has to be lower than side sleeping pillow height.
-you always have to think how you can sleep 8 hours with a pillow.  You have to neutralize your body to sleep for 8 hours. 
-both hands have to be lower than shouler lines
-can not put your arm below the pillow
-bend knees a little bit
-put your arm a little bit foreward to shoulder line
-your occipital protuberance (EOP) has to be on the center of the pillow (rectangula shape)
-if you sleep on side more than back, your pillow has to be higher.
-you can not sleep on your tummy, because you make an artificial subluxations and muscle tightness on neck and shoulder blades.
-sometimes wehn you sleep on back and you lean or tilt on sides can lead to muscle tightness on neck and shoulder blades or rib cage height differences.