“Splashing cold water on your face resets your vagus nerve.”

The research found this

The diving reflex kicks in within seconds of cold water touching the face. In a Swedish field study, 54 ninth-graders held their breath with their faces in 10°C water for about 26 seconds. Their heart rate fell by 68 beats a minute on average, from a peak near 125 to a low near 58.1Source 1 · Cross-sectional studyAssessment of arrhythmias and heart rate response in healthy adolescents performing face immersion and body submersion in ice‐cold waterLundström A, Karlsson M, Rydberg A, et al.. Physiological Reports, 2025.Read the paper ↗PubMed ↗ Eighteen of them also immersed their bodies in 2°C water. Over the first 25 seconds, face immersion cut their heart rate by 59 beats and body immersion by 7.

When cold water hits the face during a held breath, two reflexes fire at once. The vagal diving response slows the heart, and the cold shock response from the sympathetic nerves speeds it up.2Source 2 · Review‘Autonomic conflict’: a different way to die during cold water immersion?Shattock MJ, Tipton MJ. The Journal of Physiology, 2012.Read the paper ↗PubMed ↗ Six of the Swedish teenagers had extra beats from the upper chambers during face immersion, and two had ventricular bigeminy (an early beat from the lower chambers after every normal one).1Source 1 · Cross-sectional studyAssessment of arrhythmias and heart rate response in healthy adolescents performing face immersion and body submersion in ice‐cold waterLundström A, Karlsson M, Rydberg A, et al.. Physiological Reports, 2025.Read the paper ↗PubMed ↗ Lab studies that submerged young healthy people in cold water while they held their breath recorded arrhythmias (irregular heart rhythms) in 62–82% of submersions. With the head out of the water and normal breathing, the figure was about 2%.2Source 2 · Review‘Autonomic conflict’: a different way to die during cold water immersion?Shattock MJ, Tipton MJ. The Journal of Physiology, 2012.Read the paper ↗PubMed ↗

Heart-rate variability is the small change in timing from one beat to the next, and some measures of it, such as RMSSD, track vagal activity. A 2024 meta-analysis pooled 24 studies and found that these vagal measures rose after cold water or cryotherapy and stayed up for as long as 15 minutes.3Source 3 · Meta-analysisThe effects of cold exposure (cold water immersion, whole- and partial- body cryostimulation) on cardiovascular and cardiac autonomic control responses in healthy individuals: A systematic review, meta-analysis and meta-regressionJdidi H, Dugué B, de Bisschop C, et al.. Journal of Thermal Biology, 2024.Read the paper ↗PubMed ↗ In another meta-analysis, cold water immersion was the only recovery technique after exercise with a clear effect on RMSSD.4Source 4 · Meta-analysisInfluence of physical post‐exercise recovery techniques on vagally‐mediated heart rate variability: A systematic review and meta‐analysisLaborde S, Wanders J, Mosley E, et al.. Clinical Physiology and Functional Imaging, 2024.Read the paper ↗PubMed ↗

The only trial that lasted weeks randomly assigned 60 combat-sport athletes to hot-cold or neutral showers after training for four weeks. The hot-cold group finished with resting heart rates 2.1 beats lower and slightly higher RMSSD, and every effect was gone two weeks after the showers stopped.5Source 5 · Randomized trialContrast Heat–Cold Versus Thermoneutral Showering in Trained Combat Sport Athletes: A Randomized Field Trial of Recovery OutcomesHagner-Derengowska M, Trybulski R, Kruk J, et al.. Journal of Sports Science and Medicine, 2026.Read the paper ↗PubMed ↗