TOKYO – The Japanese verb “totonou” – used to describe the euphoric sensation and mental clarity associated with sauna bathing – has become a cultural phenomenon in recent years.

Researchers are now trying to scientifically explain what happens inside the body when people alternate between sauna heat, cold baths and periods spent relaxing in the fresh air while also exploring ways to make sauna use safer.

In late January, professor Kota Kodama of Tokyo’s Hoshi University visited a sauna in the capital’s Shibuya Ward to take part in an experiment aimed at uncovering the physiological mechanisms behind the experience.

Kodama, a self-described sauna enthusiast, attached blood sugar sensors and a heart rate monitor to his body before entering a sauna for 10 minutes. He then spent three minutes in a cold bath followed by a five-minute rest period. The cycle was repeated four times.

Previous small-scale studies had suggested blood sugar levels rise in a hot sauna before falling after rest following a cold bath.

Researchers believe the changes may be linked to the body’s autonomic nervous system.

Exposure to the sauna and cold bath is thought to activate the sympathetic nervous system, which excites the body and raises blood sugar levels. During the subsequent rest period, the parasympathetic nervous system – associated with relaxation – becomes more active, lowering blood sugar levels.

If the feeling associated with the Japanese-style sauna-linked cycle of “totonou”, nominated for Japan’s buzzword of the year award in 2021, is created by the shift from excitement to relaxation, Kodama believes it may be possible to identify the state scientifically by tracking changes in heart rate and blood sugar.

“I hope to show scientifically how varying sauna and cold bath durations can produce a state of complete physical and mental satisfaction,” Kodama said.

The researchers aim to collect data from more than 100 people b...