Saunas are becoming increasingly popular worldwide as a method to promote relaxation and health. Regular sauna use may have the potential to prevent neurodegenerative and respiratory diseases. In one study, men who used saunas 4-7 times a week demonstrated a 65% lower risk of developing Alzheimer's disease compared to those who used saunas only once a week. Additionally, there are reports of reduced rates of depression and lower susceptibility to colds associated with sauna usage.
While numerous studies have addressed the physical benefits of sauna use, there is still a scarcity of research from the perspectives of neuroscience and psychology. Therefore, we attempted to elucidate the "totonou" state, a state of holistic recovery achieved through sauna use, by examining brain activity and mood fluctuations.
Understanding the 'totonou' experience: Unraveling post-sauna brain activity and mood fluctuations.
The 'totonou' state refers to the intense sense of well-being experienced after a hot sauna followed by cold water, believed to be induced by a significant increase in β-endorphins. To elucidate what is happening in the brain during this state, both qualitative and quantitative data on brain activity and behavior are necessary. In this study, we conducted an auditory oddball task (measuring attention and reaction time to target stimuli) before and after sauna sessions, focusing on the fluctuations in P300 and MMN (Mismatch Negativity) components.
P300 typically appears 250-400 milliseconds after the presentation of a target stimulus. The amplitude of P300 is closely associated with attention, with larger P300 observed when attention is heightened. On the other hand, MMN emerges 100-250 milliseconds after the presentation of a stimulus and can be induced even when subjects do not subjectively attend to the auditory stimuli. Therefore, MMN is said to be elicited by auditory information processing in automatic detection of deviant (target) stimuli.
In addition to the auditory oddball task, we conducted measurements of brain activity during rest before and after sauna bathing and between each set (sauna→cold bath→outdoor exposure). To elucidate brain activity and mood fluctuations in the 'totonou' state, we implemented evaluation scales (surveys) for mood. Furthermore, we developed an artificial intelligence (AI) classification algorithm based on EEG (electroencephalogram) data and validated the classification system for states of the brain (totonou or non-totonou).
Brain Activity and Mood After Sauna: Analysis of the 'Totonou' State
As the 'totonou' state manifested during rest, the results of measuring brainwaves at this time revealed that entering the sauna gradually increased theta and alpha waves (Figure 1). This outcome closely resembles findings from previous studies measuring brainwaves during meditation.
The increase in theta waves likely reflects not only heightened cognitive and emotional processing but also an increase in consciousness and attention. Additionally, in another study on meditation, it has been shown to reduce heart rate levels. In line with this, in the current study, comparing the sauna group to a control group that did not undergo sauna sessions, the sauna group exhibited lower heart rate levels after sauna sessions. This suggests a potential similarity between the 'totonou' state and the state achieved after meditation. Furthermore, the results of a question related to emotional processing in the survey, such as feeling "isolated from everything and everyone," also aligned with these findings.
On the other hand, the increase in alpha waves may be associated with relaxation. This aligns with results commonly observed after high-intensity exercise and corresponds to the outcomes of survey items such as "feeling very relaxed" and "experiencing infinite joy." The increase in alpha energy was accompanied by a subjective experience of high happiness and positive emotions, as indicated by survey responses.
Furthermore, in the auditory oddball task conducted before and after sauna sessions, post-sauna results showed a significant decrease in the amplitude of P300 and reaction time to target stimuli (sound recognition), while the amplitude of MMN markedly increased (Figures 2 and 3). Regarding P300, as mentioned earlier, it is closely associated with attention. The decrease in P300 suggests that less attention was required to perform the task after sauna sessions, indicating a potential decrease in the demand for attention during the task.
On the other hand, the increase in MMN is generally associated with improved auditory discrimination. Therefore, the result indicating an increase in MMN after sauna sessions suggests that participants became more sensitive to auditory stimuli during sauna bathing. This, in turn, may have led to a reduced allocation of resources to attention required for auditory discrimination, potentially contributing to the decrease in P300 amplitude. These findings align with the results of the survey question on altered states of consciousness, where participants reported being able to vividly see images through memory or imagination. It suggests that sauna sessions may contribute to a clearer and more vivid mental state for individuals.
Furthermore, subjective assessment surveys revealed significant changes in physical relaxation and other indicators as a result of sauna bathing (Figures 4 and 5). From these results, it can be inferred that the 'totonou' state manifests as a combination of physical and mental emotions, including relaxation, joy, and mental clarity, accompanied by a sense of happiness and positive feelings.
The 'totonou' state in the sauna and neurofeedback.
The average performance of decoding the 'totonou' state using In-Ear EEG reached 88.34%. This indicates the potential to classify the brain's state using In-Ear EEG, marking a significant step towards future applications. In subsequent research, we plan to use AI decoding to classify 'totonou' and 'non-totonou' states, conduct neurofeedback training for users, and verify their ability to transition to the 'totonou' state. Ultimately, we aim to leverage the power of neurofeedback to achieve the 'totonou' state even without the sauna in the future.
※詳細：
Chang, M., Ibaraki, T., Naruse, Y., & Imamura, Y. (2023). A study on neural changes induced by sauna bathing: Neural basis of the “totonou” state. Plos one, 18(11), e0294137.
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