I go to bed at a specific time for the chance of enough sleep until I need to get up. If I would only sleep 3h until I wake up too early, I might have as well stayed up longer. So I stay by what I said.
Not really, I wake up when I have to. At the second. No matter if it's 6 o' clock or 9. And no matter if I fell asleep at 10 PM or 5 AM. Just because I woke up never means that I'm actually well rested.
I used to be able to do that, but after decades of sleep deprivation I must get enough sleep to automatically wake up. It was pretty neat while it lasted though.
Waking up on time allows you to do things like keep being employed and able to pay the rent on the home where you and your bed are located, highly underrated that waking up on time thing.