Yes i have been. Sauna releases heat shock proteins which are amazing for health. It lowers myostatin, lowers blood sugar, lowers blood pressure, increases hgh by over 1000% (not ****ting you it literally increase hgh by over 1000%), causes you to make more opiod receptors and up regulates endorphin productions, has shown has to prevent muscle loss post surgery, etc etc..
Biggest reason for feeling discomfort and feeling sick is, because it releases Dynorphins. Dynorphins displaces endorphins from the receptors and it binds to very stongly to the kappa opiod receptors(which is responsible for almost every negative symptoms from opiods). But this is what causes the upregulation of endorphins and the good opiod receptors.