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A study of older adults in Japan found that taking a hot-tub bath before bed was associated with better sleep quality. Participants not only felt their sleep was better, but objective actigraphy data also confirmed they slept more efficiently on average compared to their peers who did not bathe before bed. This effect was particularly strong during winter. The paper was published in Sleep Health: Journal of the National Sleep Foundation.
Sleep quality refers to how well a person sleeps, including how long it takes to fall asleep, how often they wake during the night, and how rested they feel upon waking. It is not just the number of hours slept but the depth and continuity of sleep that determine its quality. High-quality sleep involves cycling smoothly through the stages of light, deep, and REM sleep without frequent interruptions.
Poor sleep quality can result from stress, irregular schedules, caffeine, alcohol, or sleep disorders like insomnia or sleep apnea. Good sleep quality supports brain function, learning, and emotional stability. It also plays a key role in physical health, aiding immune function, tissue repair, and hormone balance. Chronic poor sleep increases the risk of heart disease, obesity, diabetes, and depression. People who sleep poorly often experience daytime fatigue, poor concentration, and irritability.
Study author Yoshiaki Tai and his colleagues wanted to explore the effects of hot-tub bathing on sleep quality under real-life conditions, accounting for various factors such as bathing behaviors, environmental influences, and individual characteristics.
Hot-tub bathing is a common evening or nighttime practice in Japan. It involves being immersed in a tub filled with water heated to 40–41°C for 10–30 minutes. The bathtub is usually designed to have a depth that allows the water level to reach the mid-thorax or neck.
The researchers note that previous studies reported that hot-tub bathing before sleep was associated with lower nighttime blood pressure and a lower prevalence of nocturia (waking up from sleep to urinate).
The study included 2,252 older adults from Nara, Japan. Their average age was 69 years, and 64% were women.
Study participants wore an actigraph on the wrist of their non-dominant hand for seven consecutive 24-hour periods. An actigraph is a device that measures movement to estimate an individual’s sleep patterns, activity levels, and circadian rhythms. They also wore a wireless device that logged their skin temperature attached to the actigraph, and another temperature logger on their abdomen (however, the abdominal one was worn for 24 hours only).
Participants completed an assessment of sleep quality (the Pittsburgh Sleep Quality Index) and kept a diary of their bathing behavior. They recorded whether they took a hot-tub bath, a shower, or did not bathe before bed, as well as the start and end times of bathing and the duration of immersion. For a majority of the participants, this diary was for one day only, but 945 of them recorded it for the full 7-day period.
The study authors used these data to estimate the temperature of the water (using abdominal skin temperature as a surrogate) and the bath-to-bed interval. Baths taken more than 5 hours and 45 minutes before bedtime were classified as not having been taken before bedtime.
The results showed that participants who took a hot-tub bath before going to bed slept better than their peers who did not. Their odds of reporting poor sleep quality were significantly lower than the odds for participants who did not bathe. The odds of poor sleep for participants who took a shower were not significantly different from those who did not bathe.
Higher water temperature during bathing and longer immersion time were associated with more efficient sleep. However, there was a negative interaction between these two factors. If the water temperature went above 41.7°C or immersion lasted longer than 18.3 minutes, further increases were no longer beneficial and were associated with reduced sleep efficiency.
Furthermore, the association between hot-tub bathing and more efficient sleep was strongest and most consistent for participants studied during the winter months. In general, subjective sleep assessments agreed with actigraphy measures, indicating better sleep for participants taking hot-tub baths.
“Hot-tub bathing was associated with better self-reported sleep quality, higher SE [sleep efficiency], and shorter WASO [wake after sleep onset] in real-life settings among community-dwelling older adults. In hot-tub bathing sessions with a duration of immersion < 18.3 minutes and proximal skin temperature during bathing (used as a surrogate for water temperature in the bathtub) < 41.7°C, a longer duration of immersion and higher maximum proximal skin temperature during bathing were positively associated with higher SE [sleep efficiency]. Our results suggest that improvements in SE, WASO [wake after sleep onset], and SOL [sleep onset latency, how long it takes to fall asleep] can be maximized when hot-tub bathing is scheduled 61–120 minutes before bedtime during winter,” the study authors concluded.
The study sheds light on the links between bathing behaviors and sleep quality. However, it should be noted that this was an observational study, and researchers did not direct participants’ bathing behaviors. Because of this, definitive causal inferences cannot be drawn from the results.
The paper, “Association between before-bedtime hot-tub bathing and sleep quality in real-life settings among community-dwelling older adults,” was authored by Yoshiaki Tai, Kenji Obayashi, Yuki Yamagami, and Keigo Saeki.
