Lots of responses here - sleeping comfort obviously a subject of interest to us 'greybeards'! smile
One tip I don't think has been mentioned (especially for side sleepers) is that your comfort can be increased on hard terrain if you sleep so as your hip lies in a slight depression. You can scoop out a shallow depression to make one if one is not already there.
As far as 'mattresses' go, I use a 3/4 length Thermarest supplemented by a light closed cell foam Ridgerest pad that I place under the Thermarest and extended to lie beneath my feet.
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Dances With Marmots