Before the APAP, were you a restless sleeper, moving around a lot in bed during the night? And is the APAP helping you sleep more soundly (up until the pain wakes you up)?
As you know, AS pain typically gets worse when you are still. So I'm thinking that if you are sleeping better now, you are probably not moving as much as you used to while sleeping. And if you are more still while you are sleeping, that may be what's causing the 4am pain.
I usually have to make one bathroom trip during the night, sometimes two. These trips actually help me sleep better because I get up and move during the night.
You need to discuss your issue with your rheumatologist and your sleep doctor. Maybe a muscle relaxer would help. Or a different sleeping position. Or a hot bath/shower right before bed.
Ginny - 58 year old female
Dx with USpA in March 2013; changed to AS in July 2015
Iritis and Scleritis, both currently in remission
unicompartmental knee replacements: right-June 2014, left-Aug 2018
MTX, Humira, Cyclobenzaprine, plus Celebrex as needed
Supplements: Folic Acid, Vitamin A, Vitamin D, Calcium, Fish Oil, Culturelle probiotic, Melatonin (as needed)