With winter approaching, thoughts in Japan turn to onsen, the luxurious hot springs that bless the entire length of this volcanic archipelago.

For the past five years, though, the onsen’s Nordic cousin, the sauna, has been gaining in popularity here. What started as the third “sauna boom” around 2019 has evolved into a lasting cultural movement. According to a 2024 survey by the Japan Sauna Institute, the number of sauna enthusiasts — or “saunners” (pronounced saunā), as they’re called in Japan — has grown by an additional 1 million in the past year alone.

Spa Metsa Otaka, a sprawling Finnish-inspired bathhouse in Nagareyama, Chiba Prefecture, houses the ostentatious “Dragon Sauna,” among other spaces. Around 35 men — naked apart from woolen hats to insulate their heads — enter a sweltering, dimly lit chamber containing four tiers of benches facing a line of heavy-duty sauna stoves. The men are vying for vacant spots on the higher benches like it’s a game of musical chairs; the higher you’re seated, the hotter it gets. Playing in the background is an ambient composition by Kengo Tokusashi, who writes music specifically with sauna sessions in mind.