I had a period a few years back where I did fasts up to 3 days quite frequently.. Once I got used to it they felt pretty good. But, the point I wanted to add is that they would have been very difficult for me if I wasn't also doing a pretty strict low-carb ketosis diet. Without the hunger control of keto and having my body acclimated to using stored fat as a primary energy source, fasts have been untenable.