RUNNING HEALTH & WELLNESS / RESEARCH REVIEW
How heat and water affect your nervous system
Warm baths, hot tubs, saunas, and gentle pool immersion produce different responses. Research suggests benefits for anxiety symptoms and sleep, but long-term effects on nerve activity and cardiovascular health remain under study.
The short answer
- Anxiety: Mixed hydrotherapy and spa trials report improvements in symptom scores, but this does not prove a hot tub treats an anxiety disorder.
- Nervous system: A small study found reduced resting sympathetic nerve firing after repeated warm baths. The study did not measure anxiety.
- Sleep and recovery: Warm bathing before bedtime has encouraging sleep evidence. Feeling relaxed is not proof of faster muscle repair.
20%
Lower resting nerve firingObserved after four weeks in nine adults. No anxiety outcomes measured.
0.46
Standardized anxiety effectPooled estimate from eight hydrotherapy or spa trials, not a percentage.
1–2 hrs
Before bedApproximate timing explored in the warm-bath sleep review.
Sauna vs. hot tub vs. warm pool: what is different?
Hot tubs and warm baths combine heat with immersion. Water surrounds the body and produces hydrostatic pressure, while heat raises cardiovascular demand and skin blood flow. Traditional saunas heat the body using hot air, generally without immersion pressure. Infrared saunas expose the body to a different heating environment and should not be treated as equivalent to traditional sauna or hot-water immersion. Comfortably warm pools add buoyancy and hydrostatic pressure, often with less heat stress than a hot tub.
| Method | Most relevant benefits studied | Important limitation |
|---|---|---|
| Warm baths or hot tubs | Acute comfort, sleep, repeated heat adaptation | Specific clinical anxiety trials are limited |
| Traditional sauna | Acute cardiovascular and thermal responses; observational well-being findings | Direct controlled anxiety-treatment evidence is weaker |
| Infrared sauna | Some passive heating and mood research | Protocols and physiological effects differ across devices |
| Gentle pool immersion or aquatic exercise | Buoyancy, confidence, relaxation, low-impact activity | Exercise and social effects are difficult to separate from immersion |
| Flotation-REST | Short-term relaxation and anxiety symptoms in controlled studies | Evidence for lasting clinical benefit remains preliminary |
A 2025 controlled crossover experiment in 20 healthy adults found that a 45-minute 40.5°C hot-water protocol increased core temperature and cardiac output more than the tested traditional and infrared sauna protocols. This was a laboratory comparison, not an anxiety-treatment trial. Read the comparison study.
What happens to the sympathetic and parasympathetic nervous systems?
Autonomic regulation: Two interacting pathways
Sympathetic pathways
Support changes in heart function and blood-vessel control during physical challenges, including strong heating.
Parasympathetic pathways
Include vagal influences on heart regulation and other resting functions. Their activity cannot be inferred solely from a change in sympathetic nerve bursts.
During hot immersion: heart rate and thermal demand usually rise. After four weeks of repeated baths: one nine-person study measured lower resting sympathetic nerve firing. Not established: a corresponding increase in vagal activity or long-term anxiety treatment.
The sympathetic nervous system supports responses to physical and psychological demands by changing heart function, vascular tone, and other processes. The parasympathetic nervous system, including vagal pathways, contributes to cardiovascular regulation and digestion. They interact rather than acting as a single switch where one simply turns on and the other turns off.
During intense heating, sympathetic and cardiovascular demands often rise. The fact that someone feels calm in a hot tub does not mean sympathetic activity immediately falls. Heart rate often increases during heating. Repeated exposure, however, has shown a different resting response.
What researchers measured directly
In a 2022 study, nine healthy adults, averaging about 59 years old, took approximately 40°C warm baths for 30 minutes, five days weekly over four weeks. Researchers measured muscle sympathetic nerve activity (MSNA) by microneurography. Average resting burst rate fell from 31.6 to 25.2 bursts per minute after the intervention. Resting heart rate also decreased. However, responses to experimental physical stressors and baroreflex sensitivity were not significantly changed. The study did not test anxiety symptoms and was too small to establish long-term clinical effects. Cui et al., Journal of Applied Physiology (2022).
MSNA is not the same as heart-rate variability (HRV). MSNA measures bursts of activity in specific sympathetic nerves. HRV is derived from variations between heartbeats and is influenced by several regulatory pathways. Neither single-number measurement provides an instant diagnosis of stress or overtraining. See our heart-rate training guide for how to interpret heart rate alongside exercise intensity and environmental conditions.
What about the vagus nerve and baroreflex?
The vagus nerve contributes to parasympathetic control of the heart and other organs. Baroreflexes help regulate blood pressure when circulation changes. Immersion and heat influence blood vessels, blood distribution, and heart rate, but it is inaccurate to claim a hot bath “activates the vagus nerve” or “resets the nervous system” based on the current intervention evidence. In the nine-person bathing study, baroreflex sensitivity did not significantly improve.
Does water immersion reduce anxiety?
A 2024 meta-analysis evaluated 17 randomized controlled trials involving 977 participants and different hydrotherapy or balneotherapy interventions. Eight trials contributed anxiety outcomes. The pooled anxiety effect was a standardized mean difference of 0.46 (95% CI 0.27–0.65), favoring the interventions. This is a moderate standardized effect on symptom scores, not a 46% reduction in anxiety. The interventions differed substantially and included aquatic exercise or spa programs, so the result should not be attributed to hot-water temperature alone. Read the 2024 meta-analysis.
Gentle floating and reduced stimulation
Flotation-REST differs from ordinary pool swimming: participants float in a quiet, low-stimulation setting. A 2024 randomized feasibility trial of 75 people with anxiety and depression found repeated flotation sessions feasible and generally well tolerated but specifically called for larger efficacy trials. Another crossover experiment in 50 healthy adults found greater immediate relaxation and lower anxiety after floating than after resting on a warm waterbed. These are promising but do not prove lasting treatment of generalized anxiety disorder. Flotation feasibility trial · Flotation crossover study.
For a runner managing stress, warm immersion is best framed as an optional supplemental practice. It does not replace evidence-based mental-health care when anxiety is persistent or disabling. For the broader connection between mental stress and training, read How Stress Impacts Your Running Journey.
Sleep, body temperature, and training recovery
Sleep is one of the most practical reasons to consider warm bathing. A 2019 systematic review found that warm water bathing or showering before bedtime was associated with better self-reported sleep quality and a shorter time to fall asleep. In the analyzed studies, a warm shower or bath approximately one to two hours before bed was a promising approach. The ideal temperature and duration for each individual remain uncertain. Read the sleep review.
Does heat help muscles recover faster?
Feeling better and restoring performance are different outcomes. Relaxation and improved comfort do not establish faster repair of muscle tissue, glycogen replenishment, or prevention of overtraining. Sleep, adequate nutrition, training load management, and recovery time retain priority. See Progressive Overload, Recovery & Overtraining, Why the No Days Off Mentality Isn't Always a Good Idea, and What Muscle Soreness Means for Training.
Blood pressure, inflammation, cortisol, and HRV
Heat exposure produces important acute circulatory changes, but a 2025 meta-analysis of 20 randomized trials found no statistically significant pooled improvement in most measured cardiometabolic outcomes, including HRV, resting heart rate, C-reactive protein, and several vascular measures. Some blood-pressure subgroup findings were favorable, with uncertainty and between-study variation. This research does not justify promises that hot tubs reliably lower cortisol, reduce systemic inflammation, or improve every cardiovascular risk marker. Hamaya et al., 2025 systematic review.
Practical ways to try warm-water relaxation safely
- Choose a comfortable setting. A bathtub or temperature-controlled pool may be preferable to intense heat when your goal is to relax, especially if you do not tolerate hot environments.
- Start conservatively. Brief sessions of approximately 10–15 minutes in comfortable water provide a reasonable starting point for healthy adults. Longer or hotter sessions are not proven to produce greater anxiety relief.
- Respect temperature limits. The CDC advises keeping hot-tub water at or below 104°F (40°C). This is a maximum, not a target temperature.
- Hydrate and exit when unwell. Leave the water if you feel dizzy, faint, nauseated, or overheated. Avoid combining hot immersion with alcohol or sedating substances.
- Adapt for health conditions. Ask a clinician before high-temperature exposure during pregnancy or if you have cardiovascular, blood-pressure, or fainting concerns.
- Track how you feel. Note sleep quality, perceived stress, and next-day energy over several weeks. Do not interpret a change in wearable HRV as proof of an autonomic treatment effect.
Use proper maintenance and disinfectant testing for shared hot tubs. CDC hot-tub safety guidance.
Common questions about heat and anxiety
Is a hot tub better than a sauna for anxiety?
Direct head-to-head clinical studies are insufficient to establish a winner. Warm-water and spa interventions have encouraging anxiety findings, but many trials combine several treatments. Choose based on comfort, access, health considerations, and personal response.
Will a warm pool have the same effects as a hot tub?
No. Temperature, hydrostatic pressure, buoyancy, movement, and sensory experience all differ. Gentle pool activity offers a lower-impact way to move and relax without seeking intense heat.
Does a lower heart rate after bathing mean lower anxiety?
Not necessarily. Heart-rate changes are influenced by many factors. A small study found changes in resting nerve activity after repeated bathing, but it did not establish clinical anxiety improvement.
Should runners use cold plunges?
Cold-water immersion creates a different acute physiological stimulus. It should not be presented as equivalent to warm-water relaxation or as a proven anxiety treatment. Training goals and individual health risks matter.
Continue reading at Confluence Running
These existing guides explain how stress and recovery fit into your overall training plan.
Primary research and sources
- Cui et al. (2022). Repeated warm water baths decrease sympathetic activity in humans. Journal of Applied Physiology.
- Effectiveness of hydrotherapy and balneotherapy for anxiety and depression symptoms: a meta-analysis. Current Psychology (2024).
- Before-bedtime passive body heating by warm shower or bath to improve sleep: systematic review and meta-analysis (2019).
- Hamaya et al. (2025). Non-acute effects of passive heating interventions on cardiometabolic risk and vascular health.
- Comparison of thermoregulatory, cardiovascular, and immune responses to passive heat modalities (2025).
- Garland et al. (2024). Flotation-REST safety and feasibility trial.
- Flotation-REST crossover study (2024).
- CDC: What You Can Do to Stay Healthy in Hot Tubs.
Confluence Running educational content. This article explains research findings and does not diagnose or treat medical conditions. Research reviewed November 4, 2026.
