Association between timing of hot water bathing before bedtime and night-/sleep-time blood pressure and dipping in the elderly: a longitudinal analysis for repeated measurements in home settings

ABSTRACT

Hot water bathing – a Japanese traditional practice – has not been evaluated for its association with night- and sleep-time blood pressure (BP) in large population. In this longitudinal analysis, bathing parameters and ambulatory BP were repeatedly measured for 2 nights in 758 Japanese elderly individuals. Participants were divided into three groups according to tertile values of time soaked in the bathtub (Duration: tertile value, 11 and 15 min), time from bathing-end to bedtime (Time before bedtime: tertile value, 42 and 106 min), and temperature of hot water in the bathtub (Water temp: tertile value, 40.3 and 41.2 °C). Participants’ mean age was 70.9 years, and mean night- and sleep-time systolic BP (SBP) and dipping were 115.1 ± 16.1, 114.2 ± 16.2 mmHg, and 14.2 ± 8.8%, respectively. Multivariable mixed-effect linear regression models adjusted for potential confounding factors suggested that nighttime SBP was significantly lower in the intermediate Time before bedtime group by 1.7 mmHg (95% CI, 0.2–3.1) and in the short group by 1.9 mmHg (95% CI, 0.1–3.7) than that in the long group. Dipping was significantly greater in the intermediate Time before bedtime group by 1.8% (95% CI, 0.7–2.9) and in the short group by 1.8% (95% CI, 0.6–3.1) than that in the long group. These associations were consistent regarding sleep-time SBP. Conversely, Water temp and Duration did not significantly associate with any ambulatory BP parameter. Remarkably, Time before bedtime significantly prolonged with increases in tertiles of Water temp (P for trend = 0.006). In conclusion, the findings of this study revealed that Japanese hot water bathing, especially the short time from bathing-end to bedtime, was associated with lower night- and sleep-time BP and greater dipping in an elderly population.

KEYWORDS:

• Bathing
• nighttime blood pressure
• sleep-time blood pressure
• dipping
• hot water
• passive body heating

Acknowledgments

We would like to thank Naomi Takenaka, Sachiko Sogahara, Keiko Nakajima, and Megumi Natsuaki for their valuable help with data collection and analysis.

Disclosure statement

KS and KO received research grant from YKK AP Inc.; Ushio Inc.; Tokyo Electric Power Company; EnviroLife Research Institute Co., Ltd.; Sekisui Chemical Co., Ltd.; LIXIL Corp.; and KYOCERA Corp. The other authors reported no conflicts of interest.

Supplementary Material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by research funding from Department of Indoor Environmental Medicine, Nara Medical University; JSPS KAKENHI (grant numbers: 24790774, 22790567, 25860447, 25461393, 15H04776, and 10124877); Mitsui Sumitomo Insurance Welfare Foundation; Meiji Yasuda Life Foundation of Health and Welfare; Osaka Gas Group Welfare Foundation; Japan Diabetes Foundation; Daiwa Securities Health Foundation; Japan Science and Technology Agency; YKK AP Inc.; Ushio Inc.; Nara Prefecture Health Promotion Foundation; Nara Medical University Grant-in-Aid for Collaborative Research Projects; Tokyo Electric Power Company; EnviroLife Research Institute Co., Ltd.; Sekisui Chemical Co., Ltd; LIXIL Corp.; and KYOCERA Corp. The funders had no role in the study design, data collection and analysis, decision to publish, and preparation of the manuscript.