> Physical tasks are obviously a different story. But your mind doesn't seem to slow down much at all for a pretty long time during fasting.

I’m not sure i agree with this. Highly productive sessions always end with me feeling exhausted despite just concentrating intently for several hours. Doesn’t your brain already consume like 30% of your energy? It would really surprise me if your brain operated at full capacity after 2+ days of no calories.

The assumption you're making is that your brain gets no calories when you have no food. Every kilogram of fat on your body is 9000 calories just sitting there. Most of us have at least 10 of em, or 90,000 calories.

While it's definitely true that some things slow down to conserve energy, it seems like the brain isn't one of em. There are moments* now I feel I'm thinking considerably clearer than usual. The world just feels unbelievably quiet. And things are getting done.

*I don't want to sell it as if it's all sunshine and rainbows. So far, getting to sleep has been hell, and the hunger pangs at times have been pretty insistent. While the hunger seems to have gone now, I have a general feeling of physical frailty, including lightheadedness when I stand up, and occasional light nausea.

> The world just feels unbelievably quiet.

This is what I sense when my fast crosses 18+ hours.

Yeah you certainly get a taste of it pretty early, and I think that taste is what made me intrigued to keep going, but lets be honest that taste is pretty well spoiled by increasingly desperate hunger. But I've just hit ~87 hours now, and everything else bad that happens mentally in the 18-60 hour period gets noticeably nicer once you go beyond the 60-70 hour mark, which makes that quietness way more enjoyable. Slept like a baby for the first time since starting last night, which makes a world of difference. Being too hungry to sleep is a special kind of hell.

This confirms some stuff I'd read on fasting. I'd read somewhere that the main pain is associated with re-establishing a new steady-state in which you're getting all your calorie needs from eating your own fat instead of your stomach contents, but I wasn't really sure if that was just wishful thinking by brain-washed fanatics. Fasting is still a non-mainstream practice so it's kinda hard to find reliable sources of information on it. But this particular point seems to be 100% legit, at least in the experience of this sample of one.

Well, I am quite skinny—it wouldn’t surprise me to be more affected by fasting than most.

Interestingly, there's some suggestion that fasting does increase your intelligence, at least for a while (at least, as always, in mice): https://www.nytimes.com/2006/12/10/magazine/10section1C.t-1....