In response to increasingly severe summer heat, KAI—the leading onsen ryokan brand by Hoshino Resorts—presents the "Cool-Down Soak" as a new standard for summer wellness. Available from June 1 to August 31, 2026, this initiative transforms the traditional hot spring experience into a strategic wellness tool designed to help the body recover from extreme heat. The urgency for such measures follows the unprecedented summer in 2025, when Japan’s average temperatures reached record highs and nationwide heatstroke-related hospitalizations exceeded 100,000 for the first time. As extreme heat becomes a serious social issue, it increasingly threatens public health by triggering sleep deprivation and disrupting the autonomic nervous system. To address these growing concerns, KAI collaborated with a balneotherapy specialist to develop a scientifically-backed bathing method. By combining this specialized soak with dedicated cooling spaces and restorative beverages, KAI provides a comprehensive wellness experience that transforms the Japanese summer into a season of recovery and revitalization.
Feature 1: The "Cool-Down Soak" – A Scientific Approach to Summer Recovery
Supervised by Dr. Shinya Hayasaka, a leading specialist in balneotherapy (onsen therapy), the "Cool-Down Soak" introduces a modern evolution of Japan’s traditional healing baths. This method centers on immersing the body for 10 to 20 minutes in "tepid waters" (around 35–36°C), a range known as the "thermoneutral zone." By soaking in water that matches the body’s natural temperature, guests can gently lower their core body temperature without the physical strain associated with hotter baths. This strategic approach offers two-fold benefits: it significantly reduces post-bath perspiration for lasting comfort and, more importantly, helps stabilize the autonomic nervous system, which often becomes disrupted by the extreme fluctuations between outdoor heat and indoor air conditioning. To ensure a safe and effective experience, KAI’s dedicated 'Yumori' (onsen masters) are available at each property to provide guidance on the ideal soaking duration, which varies by location to suit the specific mineral properties of each local spring.
Feature 2: Specially Curated Post-Soak Beverages for Deep Internal Cooling
To complement the thermal benefits of the "Cool-Down Soak," KAI offers two specially curated beverages designed to hydrate and refresh the body from within. For guests seeking a crisp finish, KAI presents its original "Post-Bath Beer." Specifically developed to be enjoyed after a soak, this craft brew features a lower alcohol content and a remarkably smooth, refreshing finish, providing a revitalizing spark to a body pleasantly warmed by the springs. For those preferring a non-alcoholic option or a nutritional boost, KAI serves "Chilled Amazake." Often referred to as Japan's traditional superdrink, this chilled, fermented rice drink is rich in essential vitamins and amino acids, making it the ideal remedy for post-bath rehydration. 
Feature 3: Immersive Post-Bath Spaces Designed for Cooling Serenity
To extend the serenity of the "Cool-Down Soak," KAI features "Yuagari" lounges—a post-bath sanctuary designed for tranquil recovery—infused with the cooling aesthetics of a traditional Japanese summer. During this limited period, these spaces are adorned with seasonal elements such as wind chimes and delicate paper fans, offering a multi-sensory escape from the heat. Guests can experience the gentle melody of regional wind chimes—a sound synonymous with cooling relief in Japanese culture—that serves to soothe the mind and lower the body's perceived temperature. Paired with the timeless ritual of cooling off with a hand-held fan, these lounges provide a nostalgic yet refined environment where guests can fully embrace the elegant simplicity of a traditional summer evening.
Program Overview: "Cool-Down Soak"
Period: June 1 – August 31, 2026
Locations: Onsen and "Yuagari" lounges at KAI properties nationwide
Reservations: Not required
Pricing: KAI Original Post-Bath Beer JPY 920, Chilled Amazake JPY 600 (*both including tax)
Note: In the event of low outdoor temperatures deemed unsuitable for the "Cool-Down Soak," water temperatures will be maintained at standard levels. KAI Aso and KAI Hakone will offer the signature beverages only. This program will not be available at KAI Kusatsu, KAI Miyajima, and KAI Matsumoto.
Supervising Specialist: Dr. Shinya Hayasaka
A prominent physician and researcher, Dr. Shinya Hayasaka has dedicated over 25 years to the medical study of bathing, onsen, and saunas, analyzing the health data of more than 70,000 individuals. A graduate of Jichi Medical University and its Graduate School of Medicine, he has held esteemed positions including Associate Professor at Hamamatsu University School of Medicine and Professor at Daito Bunka University. He currently serves as a Professor at Tokyo City University’s Faculty of Human Sciences and is the Director of the Onsen Medical Science Research Center at the Japan Health & Development Foundation. As a Doctor of Medicine and a certified balneotherapy specialist, Dr. Hayasaka has authored numerous influential works, including The Correct Way to Sauna by Top Doctors and The Doctor’s Guidebook to Hot Springs.
Contribution to SDGs
Hoshino Resorts prioritizes CSV (Creating Shared Value) management, a philosophy that seeks to align economic success with social contribution. This commitment dates back to 1917, shortly after the founding of our first inn in Karuizawa, when we began generating our own hydroelectric power using wooden waterwheels. By 1929, we had established our first full-scale hydroelectric plant to secure a sustainable energy source. Long before the global adoption of the SDGs, Hoshino Resorts was already deeply engaged in environmental management, promoting ecotourism and waste reduction. Today, we view the SDGs not as a set of external obligations, but as essential tools for advancing our CSV goals. We call our unique approach "Katteni SDGs"—a term reflecting our proactive stance in addressing social issues based on our own core values. The "Cool-Down Soak" initiative contributes to two key SDGs: Goal 3 (Good Health and Well-being), by supporting physical health during extreme heat through medical science, and Goal 13 (Climate Action), by offering practical adaptation strategies to the challenges of climate change. Through this program, Hoshino Resorts KAI continues to leverage its core business to provide meaningful solutions for a changing world.
For more information on our sustainability initiatives, please visit: https://hoshinoresorts.com/en/aboutus/sdgs/
About KAI Brand
Onsen Ryokan loved by local and overseas travelers
KAI is a leading brand of onsen ryokan where every establishment offers a contemporary take on traditional Japanese inn whilst keeping Western comforts. KAI invites one to discover the multifaceted aspects of Japan through natural hot springs, exploration of local arts and entertainment, and classic Kaiseki-style course meals using local and seasonal flavors. Experience Washoku, Japanese cuisine, presented on carefully selected ceramics for full exposure to culinary sensations.
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