Twenty-five adults who said they regularly felt stressed spent eight weeks moving through two different bath routines and one result stood out: they fell asleep faster during the phase that used neutral bicarbonate ionized water tablets. The same phase also improved questionnaire scores for sleep quality and reduced the time between waking and getting out of bed.
The study was small, it was open-label and every participant knew when the special tablets were in the tub. Even so, the design was more careful than a simple before and after test. Each person completed both routines, one with daily tablet baths and one with regular bathing, so the researchers could compare the same people under both conditions.
Researchers reported the work in a Scientific Reports study published in 2024. The team included investigators from Tsurumi University School of Dental Medicine, Stanford University School of Medicine and company-affiliated authors linked to the product being tested, which makes the findings interesting but also calls for extra caution.
For everyday readers, the useful question is narrow: if someone already takes a warm bath each night, does changing the bath water with a bicarbonate tablet add anything measurable? This study suggests the answer may be yes for sleep onset and a few related measures, but the signal came from one small group under strict instructions rather than from a large community sample living normally.
What the trial actually tested
The volunteers were adults who often felt stressed in daily life and 25 met the study criteria after screening. They were randomly split so one group started with regular bathing and the other started with tablet baths. Each intervention lasted four weeks and a one-week washout period separated the two phases.
During the test phase, participants used four HOTTAB Inc. tablets in 180 liters of warm tap water once a day. They waited at least 15 minutes after adding the tablets, kept the bath between 37 and 41 degrees Celsius, soaked to shoulder depth for at least 20 minutes and changed the water each day. During the comparison phase, they still took one full-body bath a day, but they did not add the tablets or other bath products.
The research team tracked several kinds of outcomes. Stress was measured with the Brief Job Stress Questionnaire and the Profile of Mood States. Sleep was measured in two ways: a questionnaire called the Pittsburgh Sleep Quality Index and a wearable movement monitor, or actigraph. The group had also published an earlier 2021 paper suggesting that the same bath product could boost blood flow through nitric oxide related changes, which gave this follow-up trial a proposed physical pathway to investigate.
All 25 participants completed the full crossover schedule, which is useful because dropout can distort a small trial quickly. The paper also says the statistical analysis was performed after the whole study period by a statistician independent from the study group. That detail does not solve the open-label problem, yet it does reduce one obvious source of bias in how the final comparisons were calculated.
Where sleep changed most
Sleep showed the clearest practical signal. On the questionnaire side, PSQI-J scores moved down more during the bicarbonate bath phase than during the regular bathing phase, which means reported sleep quality improved. On the device side, sleep latency improved more as well. In plain language, participants usually fell asleep sooner after getting into bed.
The study also found a better result for bed out latency, which is the time between waking up and getting out of bed. That point matters because it hints that the change was not limited to bedtime perception. The researchers paired self-report with actigraphy and sleep specialists often use activity monitors to estimate sleep patterns over several days rather than relying on a single night in a lab.
Actigraphy is still an indirect measure, so it cannot offer the same detail as full polysomnography. Yet it is a reasonable tool for a home study like this one, especially when the device method has published support, including a sleep/wake scoring algorithm and a later validation study. For readers trying to place the result, sleep latency is one of the basic ways clinicians think about how easily a person is getting to sleep.
Baseline sleep quality in the study was already poor, which makes the shift easier to read. The average PSQI-J score started above 9 in both crossover groups, then dropped to 5.8 during the bicarbonate intervention in one of the reported comparisons. Readers should not treat that as proof of a clinical cure for insomnia, but it does show the effect was large enough to show up on both a standard questionnaire and a movement-based home measure.
Stress and immune results were more subtle
The trial did not show a sweeping mental reset after a month of special baths. Instead, the stress data were mixed, with one significant advantage in the social support part of the job stress questionnaire and one significant improvement in the confusion-bewilderment score from the mood profile. Many other measures moved in a favorable direction during both bath routines, which suggests that daily soaking itself may have helped some participants.
Immune findings were more technical, but the pattern was still understandable. During the bicarbonate phase, the authors reported smaller stress-related swings in CD4+ and CD8+ T cell counts, a steadier naive to memory T cell ratio and less fluctuation in neutrophil phagocytosis. The paper treated those smaller swings as a sign that immune function was more stable under the tablet-bath condition.
Readers should keep the wording careful here. A small crossover trial cannot prove that the tablets strengthen immunity in a broad everyday sense or prevent illness. What it does show is a cluster of laboratory changes that lined up with better sleep and slightly better stress-related scores in this specific group of stressed adults following a tightly controlled bath routine.
The study also reported no significant carryover effect in the key stress comparison, which supports the decision to analyze the two periods together after the washout week. That is important in crossover research because a strong leftover effect from the first intervention can contaminate the second phase. In this paper, the authors checked for that problem rather than assuming it away.
Why the findings need caution
The strongest limitation sits in the design label itself: small randomized crossover trial, but also open-label and product-specific. Everyone knew which bath they were taking and expectations can influence sleep and mood measures even when a study uses randomization and a washout period. Twenty-five participants also leaves little room for hidden differences between people.
Funding and conflicts add another layer. The acknowledgements state that the study was funded by Cranescience Co., Ltd. The ethics disclosure says that corresponding author Ichiro Saito is a representative director of that company and receives compensation and stock ownership. Another author was affiliated with Bicarbonate Thermotherapy Institute Co., Ltd. Those facts do not erase the data, but they do raise the bar for independent replication.
Even with those caveats, the paper gives a useful, concrete result: under one month of daily home bathing, a bicarbonate tablet routine beat matched regular bathing on sleep latency, overall sleep quality and a few related measurements. The next step is clear. A larger blinded trial, ideally run by investigators without a commercial stake, would show whether the signal survives when expectation effects and company ties are pushed much lower.
One final limit is transparency around reuse. The paper says the underlying data are available from the corresponding author on reasonable request rather than through a public repository. That arrangement is common in small clinical studies, but it means outside researchers cannot inspect the raw records instantly. Independent confirmation, preferably with preregistered methods and a blinded placebo bath product, would make the result much easier to trust.
