A recent study showed that sleeping position does not influence shoulder pain. 
And, assumed ‘safe’ positions (i.e. soldier) actually have a higher prevalence of pain than assumed ‘risky’ positions (freefaller and starfish)


Holdaway LA, et al. (2018) Is sleep position associated with glenohumeral shoulder pain and rotator cuff tendinopathy: a cross-sectional study.
BMC Musculoskelet Disord.

Tristan Chai