Erin, there have been periods of time in my life when I've been able to do what you suggest - sleep until my body is done, as you say. If I did that, I'd sleep about 18 hours a day, in two phases. I'd only be awake for about three hours during business hours. It's just not feasible. I don't know why I need so much sleep now - any one of several chronic illnesses I have could be the reason. Years ago, I slept from two to four hours a night, and could sometimes skip it altogether (although I paid for it). I would love to have that smaller sleep requirement again.
My natural pattern is biphasic, I guess - when I'm not taking stimulant medications (and even most days when I am), I fall asleep sometime between 2 and 4 pm every day - no matter how much I slept the night before, no matter how interested I am in what I'm doing, no matter how much I exercise, etc. I sleep about three hours, if someone/something doesn't wake me up, although most of the time several someones/somethings do - taking a nap seems to send out a universal beacon "Call Jeni! Right now!" or "Go knock on that door over there; it doesn't matter why." lol
My conscious mind (the ambitious part of me) wants to "get away with" sleeping three hours a night (total) and spend the rest of my time being productive and connecting with people, but that's not going to happen. My body wants to sleep forever. So we compromise: I give my body half of the sleep it wants (which is still more than twice as much as I want to give it). I go to bed at the first yawn at night, and at 2 or 3 pm for a nap. I get up at the same time every day. I'm still searching for a better way, because I believe that sleeping ten hours a day is a horrible waste of time (and the 18-20 hours my body wants most days is just insane). That searching is how I found this forum. LOL And it's naptime.