I need to buy a new bed. Unfortunately I don't sleep well, never have since being a child. I can spend hours sitting up reading for about 5 hours or so on a normal night, and just napping on and off. As a result my mattress always seems to dip where I'm sitting up. I've tried good pocket sprung mattresses, memory foam, umpteen toppers, etc., but all start dipping after about 9 months and get steadily worse until I have to replace them after 2 - 3 years, and each time I do loads of research to no avail. At the moment the saggy mattress is on a good divan base, so may just change the mattress. Any recommendations from night readers please?