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Joy Hussain, Ronda Greaves and Marc Cohen of RMIT University in Melbourne and the Murdoch Children's Research Institute ran the Global Sauna Survey, published in Complementary Therapies in Medicine in 2019. The international online cross-sectional survey collected 482 valid responses from 29 countries between October 2016 and October 2017. The sample was self-selected and clearly better educated, healthier and more active than the general population: 81.8 % had higher education, 90.6 % were non-smokers and 78.8 % exercised regularly.
Relaxation and stress reduction was the most cited motive, followed by pain relief at 88 %, socialising at 85 % and circulation at 85 %. Detox was named by 83 % as a motive, although detox is one of the weakest-evidenced claims anywhere in sauna research; the survey records the belief, not an effect. 83.5 % reported better sleep, usually for one to two nights. People who used the sauna 5 to 15 times a month reported higher mental wellbeing scores than less frequent users. Reported side effects were mild, mainly dizziness and dehydration. Few respondents named cardiovascular reasons.
Everything is self-reported, the design is cross-sectional and participants selected themselves. There is no comparison with non-users; the wellbeing finding compares only more and less frequent users. Controlled sleep studies with polysomnography are largely missing.
The survey describes motives, not effects. Its main finding is the gap between what users believe and what is supported, and the sleep figures are reports from respondents, not a measured improvement.