“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, and their heart rate fell by 68 beats a minute on average.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 ↗ It peaked near 125 and bottomed out near 58. Among the 18 who also immersed their bodies in 2°C water, the first 25 seconds of face immersion cut heart rate by 59 beats, against 7 for body immersion.

The slowing runs through the vagus nerve. When cold water hits the face during a held breath, the vagal diving response and the sympathetic cold shock response fire at the same time.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.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 of cold submersion with breath-holding recorded arrhythmias in 62–82% of submersions in young healthy people, against about 2% for head-out immersion with normal breathing.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 ↗

Measures of vagal activity rise for a short while after cold. A 2024 meta-analysis of 24 studies found that vagal measures of heart-rate variability such as RMSSD 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 ↗ After exercise, cold water immersion was the only recovery technique with a clear effect on RMSSD in another meta-analysis.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 data over weeks come from hot-cold showers. Sixty combat-sport athletes were randomized to contrast showers or neutral showers after training for four weeks, and the contrast group ended with resting heart rates 2.1 beats lower and slightly higher RMSSD.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 ↗ Every effect was gone after a two-week washout.