I've tried it, but probably I don't have that "you are done sleeping" signal. When I'm awake from "long enough" sleep (10-12hr), I'm tired and lazy. When I sleep 8 hours flat, I'm still sleepy when I get up, but I'm functioning normally after morning routines (washing, breakfast etc).
One beauty of a smart phone is just before laying down to sleep, just ask your phone to set an alarm in eight hours, and put it well out of your reach. And don't get back into bed (or the couch) when you get up to cancel the alarm. Just get moving.
I have done this when I got caught up in a really good book and read until 4:00 AM. Generally, I wake up on my own, before the alarm goes off, but it is a safety valve.
I know exactly what you are talking about. It took me months to develop a proper sleeping pattern that doesnt depend on alarms. IMO thats something you need to learn, especially when you never were used to it.
It literally makes me sad how few people actually can experience this.