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Janssen, Raison, Lowry and colleagues ran a six-week randomised, double-blind trial with a sham condition at a university medical centre between February 2013 and May 2015, published in JAMA Psychiatry in 2016. Participants had major depressive disorder. 16 received whole-body hyperthermia that raised core body temperature to 38.5 degrees, and 14 received a sham treatment in the same device with fans, light and minimal heat. Staff handling screening and outcome assessment were blinded.
Compared with the sham group, the active group had significantly lower scores on the Hamilton Depression Rating Scale, and the difference persisted for as long as six weeks after one treatment. The blinding held: 10 of the 14 people in the sham group believed they had received the real treatment.
The sample is very small. The treatment used a medical infrared hyperthermia device under medical supervision, not a Finnish sauna at 80 to 100 degrees. The results are physiologically related to sauna use but do not transfer to it. The trial also says nothing about repeated sessions.
Methodologically, this is the strongest study in the field of heat and mood, because it is randomised, blinded and sham-controlled. The authors discuss thermosensitive nerve pathways to serotonergic brain regions as a possible mechanism. The trial is not a basis for a treatment promise, and it gives no reason to replace an existing therapy.