Both, but the biggest issue has been reverting to my side while asleep. I've re-aggravated my rhombus muscle a number of times doing that.
I often wind up sleeping in contorted positions, and I've had a similar issue with my rhomboid muscles.
About 6 months ago, I switched to a split keyboard and it's made a big difference for me. Sitting with my arms shoulder width apart has taken a lot of the strain out of my neck and back.