Significant fluid and electrolyte loss can occur during a 20-minute sauna session for a 225lb individual, requiring substantial rehydration.

“Regardless of what time of day you do sauna or how frequently you do it, you're going to want to hydrate after going in the sauna. When you go in the sauna, you lose water. And when you lose water, you need to replace it. [...] But in general, one way to approach this would be to make sure that you drink at least 16 ounces of water for every 10 minutes that you happen to be in the sauna.”

“Dry sauna induces sweating as part of its beneficial mechanism. Be sure to hydrate properly. In general, you might need to rehydrate with up to 16–32 oz (0.5–1 L) of fluid after a sauna session. Be sure to add electrolytes.”