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Hoshino Resorts combats summer heat with medically-supervised cool bathing programme for KAI onsen

09 Jun 2026

Hoshino Resorts has developed a “Cool-down onsen soak” programme at properties with Japanese onsen facilities – those within the company’s KAI brand.

Hoshino is promoting traditional onsen bathing as a tool for recovery in response to record-breaking extreme heat across Japan last year. 

KAI facilities will offer a medically-supervised bathing experience between June and August this year, designed to help reset the body’s internal thermostat.

The experience has been developed in collaboration with balneotherapy specialist Dr Shinya Hayasaka.

Hayasaka told Spa Business: “Hot springs are natural, so their temperatures vary, and some source temperatures are naturally low. Historically, while these hot springs are heated for use during the winter, people have long enjoyed the tradition of soaking in the lukewarm source water as-is, without heating, during the summer. In particular, bathing in water that is close to body temperature is called a "neutral bath" (fukan-onyoku), and from a temperature standpoint, it is said to be the method that puts the least amount of strain on the body. 

“Summers in Japan recently have become extremely hot, creating stress for both the mind and body, which heavily stimulates the sympathetic nervous system. The neutral-temperature bathing method that KAI proposes as a "Cool-down Bath" helps to soothe this. We are recommending this bathing method now in response to the rapid rise in summer temperatures in recent years. The Cool-down Bath allows guests to comfortably enjoy the hot spring without breaking into a sweat afterwards, and it also regulates physical condition by suppressing the activation of the sympathetic nervous system, which gets overstimulated by the heat.”

Yumori (onsen masters) at KAI properties will guide guests on bathing methods, local hot spring culture, specific qualities of the spring water and the history of each region. These specialised staff members will tailor the experience to each guest according to the season.

The tepid water temperature sits at around 35–36°C, and the recommended immersion period is 10-20 minutes. 

Guests are then encouraged to relax in Yuagari lounges, which feature regional wind bells and handcrafted paper fans to lower guests’ perceived temperature in what the company calls “sensory cooling”.

The programme is available across all KAI properties in Japan, excluding KAI Kusatsu, KAI Miyajima, and KAI Matsumoto.

The programme is part of the normal bathing facility experience and does not cost extra. Drinks designed to support the heat recovery ritual will be available at KAI Aso and KAI Hakone. These include a post-bath beer, which costs JPY 920 (US$6, €6, £5) and chilled amazake at JPY 600 (US$4, €4, £3).