I think from 16:8 to 36 is a bit too sudden of a transition. If I were you, trying to get to a point of doing 36 hours, I would add a few intermediate stages first, and build up to 36. Like do 20 for a week, then 24 and so forth, at the pace at which you are comfortable until you eventually get to 36 and it’s going to be easier. You’ve trained your brain to eat at certain hours and around the 16 hour benchmark I wouldn’t be surprised if you started feeling unwell since your body is used to getting fed around that time. You can try doing OMAD (one meal a day diet) for awhile as an alternative.