Apnea is often worse if you sleep on your back. Doctors sometimes prescribe wearing a fanny pack to prevent it.

I don't think I have it because I sleep very well on my side. But it causes back ache in the morning. Anyway - thanks for suggestion.

I get this. My default, most comfortable position is to sleep on my back, but if I do get terrible nightmares, headaches, and general shittiness.

If you sleep on your side you have no nightmares or you have no dreams at all? If the latter, I'd suspect that you don't get REM phase.