Our metabolism doesn't change in the technical sense, but in the popular sense of the word "metabolism" people absolutely slow down. At 20, missing a night's sleep wasn't even that big a deal. At 50 it is essentially impossible. I don't know what the underlying mechanisms are, but that's what people mean when they (incorrectly, it seems) say "metabolism".
Sleep quality pretty steadily declines with age - your 8 hours of sleep simply count for less than it used to, and you're actually consistently operating on decreased sleep
