The Science of Bathing: Part I

The Science of Bathing: Part I

All cultures have a tradition of it. It's as personal as it gets, yet as public as eating out. The rituals of bathing have been with us for generations and surprisingly, there's little science behind its benefits. Perhaps because the end result - cleanliness - is quite obvious. We all know a hot shower can be just as cleansing for the body as for the mind. How does a cold shower stack up to a nice warm bath? It's a bit more complex than we imagine.

Photo by Steven Diaz / Unsplash

Hot Baths

How often do you take a bath? Like many people living in urban settings, chances are you don't really have time to draw a warm bath and soak for at least 20 minutes. It's typically a 4-min shower in the morning and rush to work. But for those #selfcare Saturdays, here are some interesting scientific insights into the benefits of warm bathing.

The obvious: bathing in warm water is relaxing and increases your body temperature.
The not-so-obvious: bathing can affect blood sugar, metabolism, sleep quality and work efficiency.

When it comes to examining the effects of bathing, few do it better than Japanese scientists. A decade of research out of Japan has revealed that immersion in hot water increases core and skin temperature, and reduces blood pressure as compared to a standing shower. The vasodilation induced by hyperthermic (heating) action results in systemic elevation of oxygen and nutrient supply to the periphery, and aids in elimination of CO2 and metabolic waste (similar to what sweating does). With an increase in body temperature, heat-sensitive neurons are excited while cold-sensitive neurons are inhibited in the thermoregulatory center of the hypothalamus, causing inhibition of the sympathetic nerves and stimulation of the parasympathetic nerves, leading to vasodilatation and induced perspiration to decrease the body temperature. This is that relaxing feeling only strengthened by a glass of wine during bathtime.

A hot bath can also affect our cardiovascular system, increasing heart rate by 40%-50% and peripheral oxygen, thereby stimulating metabolism and inducing elimination of metabolic waste materials. A good soak will also help you experience the benefits of hydrostatic pressure, which induces venous flow, thereby increasing cardiac output and improving metabolism. The hyperthermic action undoubtedly relaxes muscles, softens ligament collagen and improves pain, making it a great addition for athletes and those wanting better performance.

Heat to Sleep 😴

The science of performance is also looking into the effects of showers. Recently a study on young soccer players concluded that a warm shower before lights out could improve sleep efficiency and lower sleep latency (time it takes to fall asleep) compared to those who didn't shower at night. This isn't so far fetched. Another study looking at techniques that get babies to quickly fall asleep (please fund their research) also found similarities between skin temperature and sleep onset. They compared the babys' distal (foot) temperature and proximal (abdomen) temperature, and found that more than 60% of infants fell asleep within 30 minutes if their distal-to-proximal temperature gradient (DPG) was increased by at least 2.5°C within 15 min after lights-off. In addition, if infants were still awake at 15 min after lights-off and the DPG at that time was less than -2.5°C, they were not likely to quickly fall asleep. Interestingly, skin warming has been found to increase neuronal activity in brain areas that are critically involved in sleep regulation. Warming up the baby's feet, without raising their core body temp, could be a hack to getting them to fall asleep faster. However this temperature-sleep hack may or may not work for those with clinical insomnia or the elderly.

Sweet Bath 🍭

Perhaps one of the more interesting areas of study links hot bathing to improved metabolism. One measure of metabolic fitness is blood sugar control. A 2017 study assigned 14 men to soak in a hot bath (40˚C) or do one hour of cycling, and measured metabolism. Bathing burned as many calories as a half-hour walk (about 140 cal). Sign me up! Moreover, those who took a bath and then ate had a lowered peak blood sugar level than those who exercised and then ate a meal.The key to all this may be in a key regulator to blood sugar - the heat shock proteins. HSPs are molecules that respond to stress in the body, rising after exercise and a warm bath soak (passive heating). In the long term, raised levels of these proteins may help the function of insulin and improve blood sugar control (and people with diabetes seem to have lower levels of HSPs). So it appears that activities that stimulate heat shock proteins may help improve blood sugar control, especially when exercise isn't an option.

Hot baths aren't always the best option for some. Those with cardiovascular disease could negatively impact their heart rate or blood pressure (especially when standing in the upright position) with hot water.


Back to Japan. The new bathing craze has to do with mist saunas. I've done my share of hydrotherapy in spas and haven't come across this. I believe this video shows what it is...but not fluent in Japanese so who knows! Anyway, according to the research, a mist sauna is a low-temperature type of sauna characterized by fine drops of warm water unlike the high-temperature Finnish sauna characterized by low humidity. Apparently, misting has been shown to be more relaxing than either a shower or a bath. It may even improve your work efficiency if done in the morning and can faciliate muscle recovery.

Photo by Roberto Nickson (@g) / Unsplash
These are small studies and certainly more studies are needed, but they provide interesting insights into the science of bathing. Perhaps it's not so much the temperature or water droplet size, but the ritual itself that stimulates our body and mind. Knowing that bathing is me time, can independently lower stress levels, tune up the vagal response and promote general health and wellbeing. With all the science to back a proper bath, why not take your bathtime seriously?