Hi Hesa,
It would indeed be interesting to have comparative data regarding one's phsyiological reaction that releases heat shock proteins based upon local ambient meteorological temperatures as well as the temperature contrast brought about by sauna sessions.
However, it seems that there is a heat threshold that triggers a biological reaction, and that below this threshold the manifest effect isn't reached. Beyond this threshold point one gets several benefits from staying a certain duration of time above these key temperatures. Two hours per day can apparently multiply one's growth hormone levels several fold over the course of just one week. It probably does many other things too, and hopefully most of them are positive. Perhaps some are not so good in the long term, but consderable research has been done about saunas and overall the conclusions are primarily postive. What can be said from the latest research is that it definitely changes your health profile for the better, and it therefore isn't pointless.
DD