We only sleep on a flat bed because that`s all we have ever known. The bed would have been invented hundreds of years ago when there was no electricity and no engineering. For these hundreds of years, we have slept in a most unnatural and uncomfortable position which is why we sleep on our sides, bring our knees up, and end up tossing and turning all night because we are restricting blood flow by pressing our weight on the arteries and veins.
The introduction of adjustable beds has helped to alleviate all that. Sleeping on our backs in an elevated position (almost impossible on a flat bed) is the modern approach to a better and healthier sleep allowing the body to recover from the battering of daily life