Traditional thermal therapy and hot spring bathing have proven useful for various diseases, including hypertension . We investigated the preventive effects of long-term hot spring bathing in adults aged ≥ 65 years. We found that age ≥ 85 years; history of arrythmia, stroke, gout, diabetes mellitus, hyperlipidemia, and renal disease were independently and significantly associated with a higher risk of developing hypertension during the lifetime. We found that history of chronic hepatitis; and hot spring bathing duration and time were independently and significantly protective against hypertension development during the lifetime. These results support our hypothesis that habitual nighttime hot spring bathing is protective against hypertension development.
The implications of our data can be extrapolated in comparisons regarding the prevalence of hypertension according to questionnaire survey responses in adults aged ≥ 65 years. We found that nighttime hot spring bathing, which can improve sleep disorders, might be associated with the prevention of hypertension in adults aged ≥ 65 years. In a large-scale study among an older population, Tai et al. reported that nighttime hot spring bathing is significantly associated with shorter sleep onset latency and higher distal–proximal skin temperature gradient . Sawatari et al. suggested that leg thermal therapy could improve subjective and objective sleep quality in patients with chronic heart failure . The COVID-19 pandemic has placed all people at risk for developing psychiatric and mental health disorders, including sleep disturbances . It is therefore possible that nighttime hot spring bathing may improve sleep, with may result in improving hypertension control .
According to our data, age ≥ 85 years was significantly associated with the prevalence of hypertension. Many risk factors are associated with hypertension development, such as age, obesity, family history, high-sodium diet, and physical inactivity [16, 17]. Regarding disease history, stroke  and renal disease [19, 20] are associated with hypertension. Given that health care spending on hypertension exceeded USD 70 billion in the United States between 1996 and 2016 , clinicians and researchers have great interest in proactive and preventive interventions versus reactive approaches for hypertension. In this study, we demonstrated that an alternative option for potentially improving hypertension control in adults aged ≥ 65 years is habitual nighttime hot spring bathing.
Stress has two components: an acute phase and a chronic phase . Rozanski et al. verified a direct association between cardiovascular disease and chronic stress, which is known to modulate vascular endothelial cell function and platelet aggregation . Dual stressors include psychological and physiological stressors and are known to elicit activation of the hypothalamic and sympathoadrenal axes and a subsequent greater release of stress markers such as cortisol, epinephrine, and norepinephrine, in comparison with a single stressor [24, 25]. Increased levels of cortisol and oxidative stress in the body can upregulate several proinflammatory pathways, which can result in the development of several cardiovascular diseases including hypertension . Endocrine responses to sauna bathing show that some markers of stress, such as cortisol, β-endorphins, and adrenocorticotropic hormone, respond to acute heat exposure in a highly variable manner . Different results regarding the hormone response are likely owing to differences in study methods and consideration of factors such as therapy duration, time, and frequency, which were considered in our study. Therefore, revealing the cardiovascular responses will provide a more comprehensive picture of the physiological responses to hot spring bathing. Sauna baths have decreased blood pressure compared with pre-sauna measures . Although brief exposures to sauna baths resulted in benefits for < 1 hour, including reduction of blood pressure and improvement of arterial stiffness, ≥ 3 weeks and repeated exposures upregulated enzymes and pathways, which resulted in inducing stress tolerance, stronger cellular environment, and elevated health . Due to a lack of adherence to physical activity and nutrition interventions and the time pressures among individuals aged ≥ 65 years, practical interventions that can prevent or improve hypertension, such as habitual hot spring bathing, warrant additional attention.
Our study has some limitations that should be acknowledged. First, some selection bias is expected with use of questionnaire surveys; however, bias is present owing to differences in data selection. Second, important data regarding the prevalence of hypertension was likely missing because this information was collected using self-report questionnaires. To minimize bias, we limited the inclusion criteria to age, sex, disease history, and hot spring bathing habits. Third, we have no data about the treatment and outcomes of hypertension; further studies are needed to assess the details of treatment and outcomes in patients with hypertension. Finally, the purpose of this study was to help clarify the relationship between prevention of hypertension and habitual hot spring bathing. However, it was difficult to interpret the obtained evidence, such as the interactions among duration of immersion, frequency, time, and years of habitual hot spring bathing because we did not evaluate the quality of the questionnaire data.