Hot Bath

What it is

A hot bath is a soak in warm water; the sleep studies here used water at about 40 to 42.5 °C [1]. Warm showers at about 40 °C have been tested too [2]. Japanese-style bathing means soaking up to the shoulders in a deep hot tub, usually in the evening or at night [3].

How it might work

Proposed mechanism: warm water sends blood to the skin of the palms and soles, and the body heat lost afterwards lowers core body temperature, which helps trigger sleep onset [1]. In two large field studies, people who bathed before bed had a larger wrist-to-abdomen skin temperature difference around bedtime, a sign of heat loss [4][5]. These pathways are proposed, not confirmed [1].

What the research says

The main evidence is a 2019 systematic review that pooled 17 studies of before-bedtime showering or bathing at 40 to 42.5 °C [1]. Bathing improved self-rated sleep quality and sleep efficiency, and when it was scheduled 1 to 2 hours before bedtime, as little as 10 minutes shortened the time to fall asleep [1]. The review authors call for more research on the best timing and duration [1].

Two large observational studies agree [4][5]. In 1,094 older adults (mean age 72.0), bathing 61 to 180 minutes before bed was linked to shorter actigraphic and self-reported sleep onset latency and to a higher distal-proximal skin temperature gradient around bedtime [4]. In 2,252 older adults (mean age 68.8), bathers had 1.3% higher actigraphic sleep efficiency and 3.3 fewer minutes of wake after sleep onset, and had lower odds of poor self-reported sleep quality (odds ratio 0.73) [5]. Among bathers in that study, longer immersions in hotter water did not improve sleep efficiency [5].

In a small randomised crossover trial, 11 youth soccer players who took a 10-minute 40 °C shower before lights out fell asleep about 7 minutes faster and gained 2% sleep efficiency [2]. In a 40-person pilot in healthy adults, a 10-minute sodium bicarbonate bath each evening for a week came with about 12 more minutes of total sleep than showering [6].

A 2023 systematic review of spa therapy, hydrotherapy and balneotherapy found that most of the 18 included studies reported better sleep quality on the Pittsburgh Sleep Quality Index, a standard sleep questionnaire, but the authors say more trials are needed on how it works [7]. A 2022 review of Japanese-style bathing reports that repeated hot evening bathing shortens the time to fall asleep in winter and has been linked to improvements in depressive symptoms [3].

Doses used in studies

These are the doses used in studies, not recommendations. Showers or baths at 40 to 42.5 °C, for as little as 10 minutes, scheduled 1 to 2 hours before bedtime [1]. Bathing 61 to 180 minutes before bed in the older-adult study [4]. A 10-minute shower at about 40 °C before lights out [2]. A 10-minute bath after dinner for 7 nights, with or without added sodium bicarbonate [6]. Japanese-style bathing is a longer, shoulder-deep soak in a hot tub in the evening [3].

Safety and interactions

Common effects: hot bathing raises skin temperature, and in 1,479 older adults each 1 °C rise in skin temperature during bathing went with a 2.41 mmHg higher systolic blood pressure and a 2.99 beats-per-minute faster pulse; a sudden drop in blood pressure while immersed can cause fainting [8][3]. Emergency physicians group hot baths with fainting, low blood pressure, falls, drowning or aspiration, heat illness, and kidney or electrolyte problems [9].

Drowning risk: bathtub drowning is the most common accidental death at home in Japan and mostly affects older people, usually in winter [3]. In Japan in 2014, 76% of unintentional drowning deaths in adults aged 65 and over involved bathtubs, about 4,800 older adults died from bathtub drowning each year between 2011 and 2014, and the rate rose with age, reaching 39.7 per 100,000 in those aged 85 and over [10]. Keeping the bathroom and changing room at 20 °C or warmer is recommended by the review authors [3].

Upper limits: there is no single tested safe temperature or time for healthy adults; the sleep studies used 40 to 42.5 °C for as little as 10 minutes, and long, very hot soaks are the ones linked to fainting [1][3].

Medicines and conditions: we found no studies of interactions between hot baths and medicines. People with heart conditions are among those most vulnerable to fainting and blood pressure drops during heat exposure and should ask a doctor first [9]. We found no data on liver disease or bleeding disorders.

Pregnancy and breastfeeding: in a systematic review of 12 studies (347 pregnant women), sitting in a 40 °C hot bath for up to 20 minutes did not raise core temperature to 39 °C, the level linked to birth defects [11]. However, one case-control study found that using a hot tub more than once, for more than 30 minutes, in early pregnancy was linked to higher odds of some birth defects (odds ratios from about 1.5 to above 2), and chance could not be excluded [12]. Pregnant women should ask a doctor before hot soaking. We found no data on hot bathing while breastfeeding.

Caveats and open questions

The randomised trials are small (11 to 40 people) and mainly in athletes or healthy adults, and the two large older-adult studies are observational: bathing is linked to better sleep, not shown to cause it [2][6][5]. The optimal timing and duration are still unclear [1]. The large observational studies report funding from housing, lighting and appliance companies [4][5]. No study has tested whether bathing before bed changes sleep or health over months. We found no ongoing trials of full-body hot bathing for sleep; the registered trials we found tested foot baths.