I don’t remember where I heard this, so take this with a grain of salt (no pun intended). day 1 and 2 are days that you still have some gulcose and reserve liver fats in storage for you to easily use. Once day 3 hits your easy to get to fuel is spent and you start pulling more and more from fat and your body is like “yo I I need food I’m running on reserves this fat stuff is annoying to get” then like day 4 it’s like “ok it’s not as hard as I thought” then it gets easier.
Yeah, I’ve been trying to complete a 7 day fast for over a year and always folded on day 3 because of very bad headaches. As mentioned before, electrolytes help a lot! I made sure I had them this time around and I’m on day 5, further than I’ve ever been. https://imgur.com/a/ktkbgPZ