^{Not my actual meal, just one of the plates that once upon a time was eaten at a buffet.}

After my last meal before going onto this fast, I weighed myself to see the difference after 4 days. I weighed 69.9 kilograms, and upon weighing myself today before eating, the scale showed a number of:

65.4 (kg)

Difference since starting: -4.5 (kg)

So, in 96 hours I lost 5.5 kilos when I ate nothing and drank only water. Though I can't say for sure how much of it was fat and how much just water and used carb reserves.

Effects

The good

I did a quick fitness test yesterday with pull-ups (forehand grip) and I actually managed to hit my personal record of 15 repetitions. Today before eating, I could hit 25 push-ups, also a personal record. So, I think it's safe to say that fasting doesn't brake down any muscle. In fact, according to the information I've gathered, fasting increases Human Growth Hormone (HGH) which actually helps building muscle. Of course the results I had might, and probably are, also affected by my weight loss. Fasting is also said to increase insulin sensitivity (which helps avoiding diabetes) and even increase neurogenesis in the brain. (But like said, I'm no nutritionist, so do your own research, like they say to do with crypto.)

From evolutionary standpoint, it makes a lot of sense for the body to break down fat for energy, because brain and muscle are literally the things that got the food on the table as hunter gatherers. That's why people can store a lot of fat, because with inconsistent availability of food, humans developed a "credit system" that allowed to store energy making up for the lack of food in tougher times.

The bad

For me personally, I wasn't able to sleep as much; only one night did I get the minimum recommended amount of sleep of 7 hours. Though I must admit that I went around mid-night or slightly after to bed. But even then I've been able to sleep longer, and today I woke up already at 5 am. when I went to sleep after midnight. That never happens to me.

Obviously, hunger is another "bad effect", although by changing ones perception it becomes neutral, because it is not life threatening – it doesn't need to be immediately subsidized and it is not a sign of dying per se. It is just a sign controlled by hormones, which is based on habit, not necessarily on an actual need of food.

On prolonged fasts (3+ days), the hunger tends to go away after the first few days and I noticed this too at the end of the 3rd day.

Would I do it (extended fasting) again?

Probably not, for the simple reason that losing sleep is no good. To get the benefits of fasting I will instead keep doing intermittent fasting like I regularly do and get to bed with full stomach.