Welcome to the melting pot! I am on my fourth day. Tah- dah!!!
I practice yoga too, but not the hot one. Though, I love to sweat in sauna.
Electrolytes are very important. That's why I don't fast with distilled or filtered water anymore as I used to. I drink only low-mineral content natural waters - there are some good ones in Trader Joe's. I like Italian Blue, but TJ's brand is good too.
If you feel like you have something similar to hangover on a Water Fast - it could be a good sign of low electrolytes and dehydration.
Low potassium is very dangerous. Please, read this article before you decide to fast on water: