Short read
Ten men and seven women sat in an 80 degree sauna for an hour, twice daily, across seven consecutive days. That is roughly fourteen hours of heat exposure in one week, against a normal session of 15 to 20 minutes. Leppäluoto and colleagues at the University of Oulu measured a broad hormone panel throughout, and published the results in 1986.
On day one the men showed up to a sixteen-fold rise in growth hormone. Prolactin rose 2.3-fold in men and more than four-fold in women. Cortisol and ACTH fell by the end of the experiment, while testosterone, thyroid hormones and gonadotropins were largely unchanged, which argues against a hormonal disturbance. The comparison throughout is each participant against their own baseline; there was no separate control group.
Three things rarely travel with the viral version of this number. The growth hormone response declined from day three onward, so the body adapted inside the study's own timeframe. The authors themselves raised haemoconcentration through fluid loss as a possible partial explanation. And baseline growth hormone sits around 1 ng/ml, so a fivefold rise reaches roughly 5, while deep sleep produces 15 to 20.
This is a study of 17 people from 1986, and its protocol matches no real sauna use. Secondary sources cite it with contradictory journal details and with effect sizes ranging from twofold to sixteenfold, which is why the citation itself is still being verified here. The entry exists as a correction to the most misused number in the sauna discourse, not as a promise.