Sauna use can increase blood glucose levels due to dehydration and water loss.

“Going into a sauna increases my blood glucose quite a bit. It actually spikes it as high as a meal. And then it tends to drop back down to baseline or even slightly below baseline afterwards, when I talk to people about this, somebody said, oh, it's got to be that the continuous glucose monitor was getting disrupted by the heat in the sauna. That's actually not the case. Turns out that when you go in a sauna, because you dehydrate, you're losing water, I wasn't drinking water and you're dropping a lot of water. The concentration of sugar in your blood actually goes up.”