Hello! For me, posture while sleeping on my side makes all the difference when it comes to peripheral effects, particularly pain in my elbows and my hands going numb while I’m asleep.
My shoulders tend to crunch up and in and, when I happen to wake up and notice my posture, I push my shoulders back and down and get instant relief from the pain and numbness. I have an expensive medium firm down pillow, king size so it’s long. What I try to do is sink my head into one end and hug the middle which rounds my shoulders but the the way the outer shoulder of the hugging arm tends to land keeps it crunched up toward my neck via gravity. So, when I’m aware, I shift the down in my pillow to where it raises the hugging arm up and this tends to pull it away from my neck and I get instant relief from the nerve compression in my cervical spine, the part of my spine most affected by AS.
I also am often awakened by hip pain sometimes relieved by placing a pillow between my knees which is always awkward, cuz it’s either too fat or too thin a pillow or too short to also keep the same amount of space between my ankles as between my knees and thighs. It’s also hot and I have night sweats from medications and menopause.
I guess a cooling pillow that allows for side sleep and prevents poor sleep posture for side sleepers and is long and adjustable in size and firmness somehow. Hope all this helps!