Most people already know that soaking in warm water makes the body feel more relaxed. What is far less understood is why this happens.
The comfortable sensation you feel when your body is immersed in warm water is not simply a psychological effect. There is a chain of physiological responses at work behind it, from changes in blood vessel behaviour to shifts in nervous system activity.
Understanding these mechanisms is important because it gives you the foundational knowledge to get the most out of every soak. When you know what is happening inside the body, you can adjust the duration, temperature, and your routine before and after soaking so the results are more effective.
This article explores the relationship between warm water and physical relaxation from a scientific perspective, without medical claims, but grounded in mechanisms that are well-recognised in the health field.
Here is the full explanation.
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What Happens to the Body When Exposed to Warm Water?
When your body comes into contact with warm water, several responses occur almost simultaneously.
These reactions are not something you can feel individually in the moment, but their cumulative effect creates the condition commonly known as physical relaxation.
Below is an explanation of each of the main mechanisms:
1. Vasodilation: Blood Vessels Widen
When the skin is exposed to warm water, blood vessels near the body’s surface expand. This process is called vasodilation. The widening of blood vessels increases blood flow to muscle tissue and the skin, so more oxygen and nutrients are delivered to the areas that need them.
At the same time, metabolic waste products in the muscle tissue, such as lactic acid that accumulates after physical activity, are also flushed away more efficiently through the improved blood flow. This process is similar to what happens during light exercise, but without requiring any physical effort.
This is one of the reasons why, after soaking in warm water, the body feels lighter and more refreshed even though you have not done any active movement at all.
2. Activation of the Parasympathetic Nervous System
The human body has two primary modes within the autonomic nervous system: the sympathetic mode (active during alertness or stress) and the parasympathetic mode (active when the body is at rest). Warm water helps activate the latter.
Temperature receptors in the skin send signals to the brain that the environment is safe and comfortable, prompting the body to reduce the production of stress hormones such as cortisol. The heart rate slows, blood pressure tends to drop, and muscles that were previously tense begin to loosen on their own.
This effect explains why many people feel drowsy after soaking in warm water. The body is shifting from “alert” mode to “recovery” mode.
3. Reduced Muscle and Joint Stiffness
Warm water also influences connective tissue and joint fluid. The increase in temperature makes collagen in connective tissue more elastic, so muscles and joints feel more supple. Synovial fluid in the joints becomes less viscous, which helps reduce stiffness, particularly in the back, shoulders, and neck.
If you spend a significant amount of time sitting in front of a screen, this effect can be especially noticeable.
Beyond that, there is another mechanism known as the pain gate effect. Temperature receptors in the skin, stimulated by warm water, send sensory signals to the brain faster than pain signals from tense muscles.
As a result, the brain prioritises the sensation of warmth and temporarily “closes the gate” to discomfort signals.
This mechanism explains why soreness in the back or shoulders often feels noticeably reduced almost as soon as you step into warm water.
How Long Does the Relaxation Effect of Warm Water Last?
This is a question that comes up often, and the answer depends on the duration and water temperature used.
Based on various hydrotherapy studies, the relaxation effect of warm water at temperatures between approximately 33°C and 40°C can last between 8 and 10 hours after a soaking session. This includes reduced muscle tension, lower levels of excessive alertness, and a general feeling of lightness in the body.
There is also another noteworthy finding related to warm water soaking. The more regularly a person soaks, the greater the benefit they tend to experience. A body that is accustomed to the cycle of thermal relaxation tends to respond more quickly and maintain a relaxed state for longer compared to someone trying it for the first time.
For this reason, making warm water soaking a part of your weekly routine can deliver more consistent results than doing it only on occasion.
Also Read: Morning Onsen vs Evening Onsen: Which Is Better?
Feel the Relaxation Effect of Warm Water for Yourself at Onsei
The theory behind vasodilation, parasympathetic response, and reduced muscle stiffness can be read anywhere. But feeling the sensation of a warm water soak firsthand is a different experience entirely.
At Onsei, the Signature Onsen Bath uses a handcrafted teak wood tub designed to retain warmth naturally, creating ideal conditions for the body to go through every relaxation mechanism discussed above.
You can enjoy the Signature Onsen Bath as a 30-minute add-on during any visit, or combine it with one of Onsei’s rituals such as the Onsei Flow Ritual or the Onsei Bamboo Ritual for a more complete relaxation experience.
When massage and soaking are performed in a single sequence, the vasodilation and parasympathetic responses discussed above can work more effectively because the body is already in a relaxed state by the time it enters the soaking stage.
Interested? Book your visit to Onsei in Tebet, Bintaro, or Gading Serpong now.
References
- Alim, R., & Santoso, J. A. (2025). Effectiveness of Cold and Warm Water Hydrotherapy in Patients With Chronic Low Back Pain: A Systematic Review and Meta-Analysis. Sport-Mu: Jurnal Pendidikan Olahraga, 6(2), 229–250.
- Jackman, J. S., Bell, P. G., Van Someren, K., Gondek, M. B., Hills, F. A., Wilson, L. J., & Cockburn, E. (2023). Effect of hot water immersion on acute physiological responses following resistance exercise. Frontiers in Physiology, 14, 1213733.
- Dehghan, M., & Farahbod, F. (2014). The efficacy of thermotherapy and cryotherapy on pain relief in patients with acute low back pain. Journal of Clinical and Diagnostic Research, 8(9), 01–04.
- Nascimento, P. R. C., Costa, L. O. P., Araujo, A. C., Poitras, S., & Bilodeau, M. (2023). Effectiveness of hydrotherapy in the management of chronic low back pain: An updated systematic review and meta-analysis. Archives of Physical Medicine and Rehabilitation, 104(5), 957–971.
