I go in with a full water bottle of water and BPN electrolytes and drink the entire bottle during my session.

Here is a short clip on a sauna protocol after resistance training—pretty interesting stuff.

Hi! Staying hydrated (with water + electrolytes) is super important. I always have my bottle outside the sauna and rehydrate before/after rounds. I have seen people bring bottles into the sauna before; if you do just choose one with material that will be safe to use in the heat. I've never heard talk of drinking too much water affecting the health benefits of sauna (of course, hyperhydration is a health concern under any circumstances, with or without sauna), but I can imagine this could be relevant for someone with specific training goals in mind. For me there was a bit of a learning curve figuring out how much I had to consume and when to feel my best. That's what I'd recommend personally, in addition to consulting a doctor if you have specific health concerns and some type of sports medicine expert/coach if you have specific performance goals.