[k__]

I bought a hard (4) matress last year. Because a doctor said the harder the better. Now, I have low-key backpain, can't recommend.

[truth_]

Actually I have been sleeping on a hard mattresses for years. They are/were barely softer than the floor.

Never had any back pains.

And my father's doctor agrees with your doctor, and I have had this kind of bed since my early childhood.

I have trouble sleeping in soft beds of hotels and other places.

[k__]

I don't sleep on my back. Do you think this has anything to do with that?

[truth_]

I have many problems with my postures thoughout the whole day (bad habits and WFH), but I exercise semi-regularly.

I do stretching, running, or swimming- one or multiple of these all year.

I guess that is what saves me from back pains and other trouble despite bad postures.

Even when I didn't exercise (before age 14), I did not have any back pains despite sleeping on a very hard mattress.

Talk to your doctor and/or do a little exercise regularly. That should help. Don't start exercising with existing back pain without consulting an expert, though.

[k__]

It's not much and mostly right after sleep.

So, yeah, I'll do more training.