In a recent study published in the journal PLOS One, researchers investigated the neural impacts of the Japanese sauna practice called 'tononou'.
Study: A study on neural changes induced by sauna bathing: Neural basis of the “totonou” state. Image Credit: r.classen/Shutterstock.com
They evaluated the physiological, neural, and mood changes in twenty adults following three alternating sets of hot sauna and cold water, followed by rest.
Analyses of participants' subjective emotional state and brain activity revealed that saunas significantly increase neural alpha and theta powers, improving attention and brain efficiency.
Behavioral tasks were much easier to complete in the post-sauna state than before the sauna intervention. Researchers finally developed an artificial intelligence model for brain state classification, which achieved more than 88% accuracy in neural state identification.
What is totonou?
A totonou (tonneau) is an outdoor, compact, circular sauna that closely resembles a large barrel turned on its side. Like most other forms of sauna, the totonou process involves immersion in a steam or hot air bath (70°C to over 110°C) followed by cooling off in snow or cold water.
People sensitive to cold water use an outdoor rest period as a substitute before re-entering the sauna. The science behind totonou involves activating the intrinsic thermoregulatory physiologies of homeothermic (warm-blooded) organisms, including mammals and birds.
Despite potentially originating in Finland, commonplace in most households, the Japanese incorporated the totonou as a substitute for their traditional hot spring sauna routine. They popularized using between three and five repeats of sauna-cold water-rest during a single sauna session.
Previous research, most notably the long-term, ongoing Kuopio Ischemic Heart Disease (KIHD) Risk Factors Study, has highlighted the age-related disease benefits of saunas, especially the neurodegenerative effects of advancing age. Participants exposed to between four and seven weekly sauna sessions were found to have a 65% reduced Alzheimer's risk than controls.
Recent studies have attempted to understand the mechanisms governing these benefits and have hitherto revealed that sauna exposure promotes the transient release of β-endorphins and growth hormones, associated with decreased depression and other mood disorders. Separate studies on athletes have presented increased immune cell counts and improved subjective mood.
Unfortunately, neuro- and psychology-based research into sauna's effects remains lacking. Revealing the neural and psychological benefits of the therapy would help better inform medical professionals and the general public.
It may pave the path to saunas and tononou being suggested as a non-invasive clinical intervention for treating physiological, neurodegenerative, and mood conditions.
About the study
In the present study, researchers attempted to corroborate previous work on the benefits of totonou, and elucidate the impacts of the therapy on subjective mood and objective brain activity. They further developed and tested an artificial intelligence (AI) based electroencephalography (EEG) classifier to evaluate the psychological effects of totonou before and after sauna exposure.
The study cohort comprised 20 healthy adults (14 men) between the ages of 21 and 41. Inclusion criteria involved the complete lack of any hearing or speech impairments.
Additionally, to avoid behavioral evaluation biases, all participants were selected for being right-handed. Eight men and two women were assigned to the case (sauna) group, and six men and four women were assigned to the control (no sauna) cohort.
The sauna test comprised three sets of pre-sauna, sauna, and post-sauna, all involving heart rate (HR), EEG, salivary a-amylase activity (SAA), and auditory oddball tasks. During the rest phase, in-ear EEG was performed with verbal questionnaire presentation.
The sauna intervention consisted of a sauna (85~90°C), water bath (16°C), and rest (20°C) carried out for 10 min, 2 min, and 7 min, respectively. In control, the intervention comprised warm water baths (37°C), followed by the cold water and rest phases.
A multivariate analysis of variance (ANOVA) was used to measure differences in scalp EEG and in-ear EEG recordings between case and control cohorts. Subjective moods recorded using the questionnaire were compared using a two-way repeated measurements ANOVA. The SAA test was used to measure participants' stress hormone levels objectively.
Data from these results (90%) was used to train the AI model, using Linear discriminant analysis (LDA) with the least absolute shrinkage and selection operator (LASSO).
Following training, the remaining 10% of the data (first) and all (randomized) were used for model validation and performance evaluation.
The present study revealed that P300 EEG amplitude reduced significantly between the pre-and post-sauna stages, suggesting improved relaxation and physiological attention efficiency. Mismatch negativity (MMN) amplitude increased, reflecting an increase in pre-attentive auditory cognition and improved sound discrimination.
"However, the RT of participants in the sauna group who showed increased MMN amplitude in the post-sauna phase shortened compared to before the sauna. This suggests that participants became more sensitive to auditory stimuli throughout sauna bathing.’’
Neural oscillation analyses revealed that each sauna set corresponded to a significant increase in cases' alpha and theta power (with beta remaining unchanged), representing results similar to (but outperforming) previous studies on the beneficial impacts of meditation.
Theta activity is associated with efficiency in cognitive and emotional processing, while alpha activity indicates increased relaxation, thereby allowing for additional mental resource allocation of emotional and cognitive processing. Notably, these objective results mirror the subjective emotional feelings of participants, as revealed from questionnaire analyses.
"From these results, we think that the "totonou" state is manifested in physical and mental feelings of relaxation, pleasure, mental clarity, accompanied by well-being and/or positive emotions. Changes in brain activity were reflected by increased theta and alpha power, decreased P300 and increased MMN amplitudes in the auditory discrimination task. The increase in MMN indicates increased activation of the pre-attentional auditory process, leading to a decrease in attention-related brain activity P300. This indicates that the brain is in a more efficient state.’’
Analyses of behavioral task efficiency were significantly higher in the case group when compared to the control and within the case group (pre-sauna versus post-sauna), highlighting the real-world benefits of sauna and totonou therapy.
While the neurodegenerative benefits of totonou could be ascertained from this short-term study, these results suggest that totonou can provide immediate benefits in cognitive performance and 'feelings of better moods' across adult age groups.
The novel AI engine developed during this study was found to classify brain states (using EEG data) with an average accuracy of 88.34%, making it an ideal future tool in the psychologist's arsenal to objectively measure mood and cognitive performance.
The present study aimed to investigate saunas' psychological and cognitive benefits, both from the objective scientific standpoint and the subjective emotional participant angle. Results indicate that the totonou state is characterized by physical and mental clarity of mind, relaxation, increased happiness, and/or positive emotional responses.
"We further observed increased theta and alpha power as well as decreased P300 and increased MMN amplitudes. This manifests in objective physical changes (decreased HR) and cognitive responses (shorter RT). Finally, the average accuracy of classifying brain states using our AI decoding algorithm was 88.34%, having potential for future application."
Together, these results highlight the benefits of sauna therapy and propose that saunas can be used as non-invasive clinical interventions across all conditions pertaining to cognition and mood.