I am not a doctor, but I still suspect that your body is catching up on sleep it thinks it needs and the bad feeling after it is due to just switching between modes (switching off your current 5-7 hrs of sleep a night). If so, you will likely need way more than 1 night to transition.
As a personal anecdote: I once slept for 10.5-11 hours a day for about 2 weeks. I decided that I need to catch up on sleep, so I went to bed at 8PM every night, fell asleep by 9 and awoke between 7:30 and 8. After about 2 weeks I stopped falling asleep by 9 and would fall asleep around 11 and wake up at 7, at which time I stopped my "sleep fattening". I did feel like a weirdo going to bed at 8, but it felt great at the end.
YMMV, but if you can sleep for 12 hours a night I would consider letting your body do it for at least several days to see if it would switch to a different mode that your would like more. My 2c.
Another personal anecdote to support this; I’ve done this sort of “reset” more than once, and there is a qualitative difference between getting 9 hours of sleep between 10pm-7am and 1am-10am for me. Your mileage may vary, but that 3 hour shift is the difference between feeling like a winner or feeling like a loser for me. I don’t know if it is psychosomatic, but I also don’t care if it is as long as it keeps me feeling good.
As a personal anecdote: I once slept for 10.5-11 hours a day for about 2 weeks. I decided that I need to catch up on sleep, so I went to bed at 8PM every night, fell asleep by 9 and awoke between 7:30 and 8. After about 2 weeks I stopped falling asleep by 9 and would fall asleep around 11 and wake up at 7, at which time I stopped my "sleep fattening". I did feel like a weirdo going to bed at 8, but it felt great at the end.
YMMV, but if you can sleep for 12 hours a night I would consider letting your body do it for at least several days to see if it would switch to a different mode that your would like more. My 2c.