When clients show up with neck or shoulder pain, one question I ask is, "what position are you sleeping in?" Often, they are sleeping on their stomachs with their arm up under their pillow and their head. having the neck twisted to one ide while sleeping, plus the shoulder in that position can aggravate both, leaving the tummy sleeper with neck an shoulder pain.
I generally suggest to try to sleep one ones back, face up, which keeps the body in a more natural position. Yet, many clients state they do try to sleep that way, but wake up in the same old position of either stomach or side sleeping.