A 30-minute sauna session, with a brief cooling period under a cold shower midway through, increased the number of all circulating white blood cells. Neutrophils and lymphocytes, which are central to immune defense, returned to baseline within half an hour.

“This may indicate that sauna bathing mobilizes additional white blood cells into the bloodstream from tissues, where they are then redeposited after the session. This kind of periodic release of white blood cells into the bloodstream is beneficial, as once they leave their storage sites, they are better able to patrol the body and respond to pathogens,” says Ilkka Heinonen, an Academy Research Fellow at the University of Turku.