If your Parkinson’s is very advanced turning in bed can be difficult. A memory foam or a mattress you sink into can make it harder to turn. Firmer is better for turning but you want to be comfortable too so you need to balance your specific comfort so you get the best sleep.

I've got a Sertapedic mattress it's firm but comfortable and very heavy and costly

I recommend considering a hybrid mattress that has both inner springs for support and the memory foam for comfort. When you do shop for a mattress make sure you sit on the edge to see if it has true support, it should not compress down to your frame. For me I went with a medium firm mattress with an adjustable base and it has really made alot of difference even tho no matter what most of us regardless struggle to get sleep thru the night..good luck!!

Yes I agree firmer is better I just got rid I’m my pillow top for a firmer mattress!!!