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Many people try to take baths for fatigue recovery and detox diets, and purchase bath additives and bathing products, but perhaps not many people pay attention to bathing for deep sleep.
This time, we will introduce the best time and method for bathing for a good night's sleep, as well as how to use showers effectively.
Effective bathing methods for sleep
For a good night's sleep, it is recommended to take a bath 90 minutes before going to bed. Although basal body temperature varies from person to person (*1), taking a full body bath at around 40 degrees Celsius is said to be effective in warming the body to its core.
Some people prefer hot baths, but hot baths activate the sympathetic nervous system and promote wakefulness, which can negatively affect subsequent sleep. In addition, hot baths cause a rapid increase in heart rate, so it is advisable for people with high blood pressure, the elderly with reduced thermoregulatory function, and infants with weak immunity to set the water temperature to a milder setting.
Reason for bathing 90 minutes beforehand: The key is raising and lowering body temperature.
There are "skin temperature" and "core body temperature," and core body temperature is said to be up to 2°C higher than skin temperature. For sleep, it is important that the difference between skin temperature and core body temperature narrows, that is, skin temperature is ultimately high and core body temperature is low. To achieve this, it is necessary to first increase blood flow to the skin, and then dissipate heat from the skin to lower core body temperature.
To ensure smooth and high-quality sleep, there is a method of intentionally raising core body temperature before going to bed. The reason for raising core body temperature is that core body temperature tends to drop significantly after it has risen.
As an easy method, taking a bath before going to bed can be said to be the most effective.
The human body is regulated by homeostasis to prevent significant changes in body temperature. The change in skin temperature due to bathing is an increase of approximately 0.8 to 1.2 degrees. Core body temperature is strongly affected by homeostasis and does not fluctuate easily, but bathing can significantly contribute to its fluctuation. Experiments have shown that core body temperature increased by approximately 0.5-0.6 degrees after bathing in 40℃ water for 15 minutes.
The core body temperature, which temporarily rises sharply after bathing, attempts to return to its original temperature through homeostasis, thus lowering body temperature. However, a characteristic of this process is that the body temperature tends to drop significantly in proportion to how much it has risen. Therefore, bathing can more effectively lower core body temperature.
An important finding from actual experiments is that it took 90 minutes for the body temperature, which had risen by approximately 0.5-0.6 degrees, to return to its original level and begin to drop. Therefore, when taking a 15-minute bath at 40°C before going to sleep, it is necessary to finish bathing 90 minutes beforehand.
By bathing approximately 90 minutes before your planned sleep time, you raise your skin and core body temperatures. Subsequently, heat dissipation, such as sweating, causes your core body temperature to start dropping, returning to your original temperature around your planned sleep time. As your core body temperature continues to drop further, and the difference between your skin and core body temperatures narrows, you naturally feel sleepy and achieve deep sleep.
Secrets to further enhance the effect
A bath with water at around 40 degrees Celsius is common, but you may add bath salts according to your preference or visit hot springs while traveling. To what extent do these affect sleep, other than their efficacy and relaxing effects?
To investigate the effect on sleep, a study was conducted on changes in body temperature when taking a regular bath, a sodium spring bath, and a carbonated spring bath, all at 40 degrees Celsius (*2).
The study found that the increase in core body temperature during bathing was greater in hot spring baths such as sodium spring and carbonated spring baths than in regular baths. Furthermore, after heat dissipation, the decrease in core body temperature was also greater in hot spring baths such as sodium spring and carbonated spring baths than in regular baths.
Furthermore, in hot spring baths, the amplitude of non-REM sleep in the first sleep cycle, which is the most important 90 minutes after falling asleep, was greater, meaning that sleep was deeper. However, sodium spring baths have disadvantages such as causing fatigue after bathing, such as heat exhaustion or dizziness, and stinging fresh wounds.
Considering the above, carbonated spring baths, like regular baths, do not cause bath fatigue, so carbonated spring baths are generally the most recommended.
This is especially recommended for those who wish to visit hot springs daily, the elderly, and athletes. While carbonated spring baths are generally the most recommended for sleep, hot springs such as sodium spring baths each have their own unique benefits, depending on whether you are on vacation. It is best to use them according to your purpose, such as for injuries, fatigue recovery, or skin beautification.
How to deal with wanting to sleep quickly
There may be times when you're busy and tired, and just want to go to sleep right away. If you don't have time to soak in a tub and finish bathing 90 minutes beforehand, but still want to remove your makeup, wash your hair and body, and feel refreshed before sleeping, then a shower is recommended.
Experimental data shows that while bathing in 40°C water for 15 minutes increases core body temperature by approximately 0.5°C, soaking in milder water below 40°C or taking a quick shower suppresses the rise in core body temperature. Therefore, the time it takes for the temperature to return to normal is shorter, and drowsiness sets in earlier than the 90 minutes required for a bath, but the strong sleep-inducing effect of a half-body bath cannot be expected.
However, since the increase in core body temperature is less with a shower compared to a bath, the decrease is also less, and the degree of sleep pressure is inferior to a bath. Therefore, when you have time, it is advisable to try to take a bath 90 minutes beforehand.
【References】
*1: What's the ideal bath temperature?, O-Furo No Sommelier SHOP!
*Reference: Japan Hot Spring Research Institute
https://www.rakuten.ne.jp/gold/bathlier/b-cafe/ofuro_recipe/ofuro_recipe4/ofuro_recipe4_4.html
*2: (Joint research with Dr. Kambayashi and Dr. Uemura from Akita University, former and current members of SCN Lab)
