Due to a random medical issue (which has since become much more manageable), I got bad chest pain from sleeping on my side.
Despite this, I absolutely couldn't get a good nights sleep on my back, despite months of having to to. I end up being awake until I fall asleep from exhaustion and then waking up again after a few hours. It caused a lot of other problems as I was only getting between 3-6 hours sleep for weeks.
Months later when my condition improved, I went back to side sleeping, and even when it was still causing me pain, I slept much, much better.
Just an anecdote and I'm sure people are unique for things like sleep, but my own personal experience is that it isn't easy to switch.
That said, I actually like a hard mattress despite being a side sleeper.
It's all anecdata, but I think people are just different.
If I sleep on a too firm mattress, I confirm my shoulder starts to hurt during the night. When I’m sleeping, I can rest for several hours, even the whole night, without moving or changing side.
But on firm mattress I wake up several time during the night to change side.
About the orientation, it’s really strange, and I cannot explain why, but it depends. And I mean the side is VERY important, as I can’t fall asleep if I’m on the « wrong » side.
But it’s not always the same side. And sometimes it lasts for weeks or even month. And one day I switch for no reason I can find, because it’s impossible to fall asleep on the side I’ve spent weeks sleeping on.
Strange.
I have a firm mattress because i had a sore back, which in turn caused me to wake up tired.
A sitting non physical profession is the cause of all this. Sometimes i do have a sore back, but going to the (climbing) gym fixes it for weeks.