May 1, 2024

Daily Bathing in Bicarbonate Water Baths Improves Stress, Sleep, and Immune Function

Results from a randomized crossover trial
Surprising benefits from warm, bicarbonate-enriched baths.


Ushikoshi-Nakayama R, Yamazaki T, Omagari D, et al. Evaluation of the benefits of neutral bicarbonate ionized water baths in an open-label, randomized, crossover trial. Sci Rep. 2024;14(1):1261. 

Study Objective

To determine the effects, if any, of neutral, bicarbonate, ionized water (NBIW) bathing vs regular bathing on mental stress, sleep, and immune function

Key Takeaway

Daily NBIW bathing led to improved sleep both subjectively and objectively in a group of physically healthy participants.


Open-label, randomized crossover trial


Of 41 potential participants, 25 met the inclusion criteria. Criteria included healthy people aged 34 to 56 years, with a mean age of 49.4 ± 6.4 years, who regularly felt “stressed.” Ten men and 15 women completed the trial.

The exclusion criteria were extensive, ensuring the participants were physically healthy and able to safely carry out the bathing in either arm. Details on exclusion can be found as an open-access supplement to the study online. 


Composition of bath salts used in NBIW bathing: 75% sodium bicarbonate, 5% sodium carbonate, 15% citric acid, and 5% “other.” Four 15-g tabs were used in each bath. 

Participants were randomized to 2 groups. Each group underwent 2 intervention periods: In each period, participants took a warm (37–41 degrees Celsius), full-body bath daily for at least 20 minutes for 4 weeks, with a 1-week washout period in between. 

In 1 group, participants first took a “control” bath (with nothing in the water), and in the second period, they took a NBIW bath. In the second group, participants took baths with bicarbonate salts during the first period and then control baths the second period.

Study Parameters Assessed

  1. Investigators evaluated job stressors, stress reactions, and social support using the Brief Job Stress Questionnaire (BJSQ).
  2. They assessed subjective sleep quality using the Japanese version of the Pittsburgh Sleep Quality Index (PSQI-J).
  3. Investigators assessed objective sleep quality using an activity meter that participants wore around the waist at all times except when bathing. This meter collected data that investigators analyzed to evaluate bedtime, total sleep time, sleep latency (time it takes to fall asleep), wake after sleep onset, sleep efficiency, time out of bed, and bed-out latency (time it takes to get out of bed).

Primary Outcome

The primary outcomes were mental stress as assessed with the BJSQ, sleep as assessed by the activity meter, and subjective sleep quality as assessed using the PSQ-J. 

Secondary outcomes included flow cytometry testing of immune-cell subsets and circulating cytokines using enzyme-linked immunosorbent assay (ELISA).

Key Findings

While subjective stress measures improved in both groups, NBIW bathing resulted in greater improvement in the social-support category. 

Similarly, while sleep improved with both groups, the PSQI-J showed an increased subjective quality of sleep with NBIW bathing. 

Objective sleep metering also showed a significant reduction in sleep latency and bed-out latency with NBIW bathing, so patients were able to fall asleep faster and get up faster in the morning.

Secondary outcomes of the study included measurement of immune-cell subtypes before and after the intervention. The authors propose that NBIW bathing decreases stress, which positively impacts immune function. 


In the “Conflict of interest statement,” the authors write: “IS [Ichiro Saito] a representative director of Cranescience Co., Ltd., Tokyo, Japan, and receives compensation and stock ownership. The other authors declare no competing interests.” 

Practice Implications & Limitations

Many of our patients have high levels of stress, are under-rested, and have trouble getting good quality sleep. Over the long term, overproduction of stress hormones increases inflammation in the body and can contribute to chronic issues such as cardiovascular disease, blood sugar dysregulation, and cancer.1 In contemporary Western society, we lack regular rituals of rest that help to engage the parasympathetic nervous system and promote healing and rejuvenation of the body. Regular warm-water bathing, or balneotherapy, is a noninvasive, low-cost way to create such a ritual. Further, this study shows us that adding bicarbonate to bathing could amplify this effect. 

People throughout the world have used balneotherapy for many centuries for its relaxing, healing powers. In more recent times, it has been found to reduce cortisol levels, improve stress resilience, and improve the quality of sleep.2-4 In vitro studies on human and animal groups have also demonstrated that thermal waters exert anti-inflammatory and immunomodulatory effects. This includes decreases in pro-inflammatory molecules such as tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1β), and C-reactive protein and an increase in anti-inflammatory molecules.5

With the understanding that balneotherapy exerts positive effects on stress, sleep, and immunity, the authors of this study have focused specifically on bicarbonate-enriched waters for bathing. “Soda” waters found at natural hot springs have been particularly revered throughout time and are advertised at different hot-springs resorts as possessing the power to lower blood pressure, improve type 2 diabetes, aid in digestion, improve cardiovascular health, and more. In this study, the authors created a consistent concentration of bicarbonate to quantify the specific mental and physiological benefits of NBIW bathing. 

The authors postulate that this is related to increased blood flow to the brain, improving cognition and easing stress.

The addition of bicarbonate yielded additional benefits in stress reduction. While subjective measures of job stressors and stress reactions improved with control bathing and NBIW bathing, a significant improvement in “social support” was found only with NBIW bathing. This is interesting because participants perceived that their support networks improved after only the addition of a daily bicarbonate bath, which, if this result is valid, implies that there is some type of biochemical effect that is changing thought patterns. The authors postulate that this is related to increased blood flow to the brain, improving cognition and easing stress. The authors performed an earlier study that showed that bathing in NBIW leads to percutaneously absorbed carbon dioxide. The carbon dioxide changes to bicarbonate ions, which act directly on endothelial cells to increase nitric oxide (NO) production via phosphorylation of endothelial nitric oxide synthase (eNOS). This, in turn, dilates blood vessels and improves blood flow.6

This same mechanism likely contributes to the positive impacts found in this study on sleep. While balneotherapy generally has a positive impact on sleep, the NBIW intervention showed a significant decrease in the amount of time it took to fall asleep as well as get out of bed in the morning. A similar study published last year found similar improvements in sleep quality with bicarbonate-enriched bath water; it showed a faster elevation of body temperature when bathing in enriched water than when bathing in a control solution and improved chills and sleep quality.7

Finally, this study had intriguing results in the secondary end points of immune-cell mediation. As mentioned above, balneotherapy generally has been associated with a decrease in pro-inflammatory mediators and an increase in anti-inflammatory mediators, which can have a positive impact on immunity and cancer prevention. In vitro studies have found that high sodium bicarbonate in a culture medium increases the expression of IL-6, inducible nitric oxide synthase, and cyclooxygenase-2 genes, as well as TNF-α production in activated macrophages.8,9 This study measured white blood cell subtypes and found a decrease in CD8+ T cells and the proportion of CD16+ /CD56- cells, as well as an increase in neutrophil phagocytosis. These are indicators of decreased stress and increased anti-tumor activity.

The final question for us regarding this study is: How can we help our patients implement this in their daily routine? A quick Web search shows the only products similar to that used in the study are produced in Japan (where the study was performed) but can be found on eBay and Amazon. Because of the added citric acid and sodium carbonate, I do not know if simply adding baking soda to the bath would have the same effect. But for our patients with severe insomnia, trying bicarbonate in the bathtub or sourcing these tablets to add to their sleep-hygiene routine would be a low-force intervention that would probably be worth trying.

Conflict of Interest Disclosure

The author declared no conflict of interest.

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