I was curious about this as well, and wondering if the aches the morning of day 3 were from that. I wonder if it's different since it was a slow process and maybe my body had a chance to liberate the needed electrolytes from metabolizing whatever it was for energy while I was fasting. Or maybe the process of sweating removes the electrolytes faster?
I'll have to look into detail on longer fasts I'm sure you would need to be mindful and likely replace your electrolytes.