The Vagus Nerve and HRV: What the Science Actually Shows

The vagus nerve is having a moment.

It’s being positioned as a master switch for stress, inflammation, and longevity. But how much of that is physiology and how much is hype?

The vagus nerve is the superhighway of the parasympathetic nervous system. It starts in the brainstem, travels down the neck (one branch alongside each carotid artery) and then into the chest and abdomen. The vagus nerve is actually ~200,000 fibers innervating the heart, lungs, GI tract, liver, and pancreas. Let’s look at its functions:

1. Rest and digest

The vagus nerve acts as a direct brake on the sympathetic "fight or flight" system. Vagus nerve activation slows the heart, stimulates peristalsis of the GI tract and secretion of enzymes to aid digestion.

2. Motor and Special Sensory Functions

The vagus nerve controls physical structures in the neck and throat responsible for speech and swallowing. It carries taste sensations from the back of the tongue and epiglottis as well as skin sensations from the external ear.

3. Gut-brain axis

Approximately 80% of the fibers in the vagus nerve are sensory, which means they listen to what’s going on in the body. It monitors blood pressure, blood glucose levels, and the presence of nutrients or toxins in the gut. It detects metabolites produced by gut bacteria (like butyrate) and sends signals to the brain that influence mood and satiety.

4. Inflammation control

The vagus nerve detects inflammation in the gut and releases acetylcholine which suppresses cytokine production (i.e. TNF-alpha). The vagus nerve can put the brakes on the immune system. If you cut the vagus nerve in a mouse, you can see increased inflammatory cytokines and even signs of inflammatory bowel disease (i.e., Crohn’s disease).

By contrast, if you surgically implant a vagus nerve stimulator, you can see reduced inflammation in rheumatoid arthritis patients (reference). We are just beginning to understand the power of the vagus nerve.

Given all of these important functions, we want our vagus nerve to be working optimally. Is there a way to measure vagus nerve activity?

Heart rate variability

Heart rate variability (HRV) is the variation in time between heartbeats measured at rest or first thing in the morning. If your heart rate is 60 bpm, it isn't a perfect metronome. There might be 1.05 seconds between one beat and 0.95 seconds between the next. This variability is created by the vagus nerve firing intermittently to prolong the intervals. Higher HRV is a sign of a responsive, active vagus nerve.

So HRV is produced by vagus nerve activity in the heart. Higher HRV is believed to be good, because it is a proxy for vagus nerve activity. It does not necessarily mean the vagus nerve is also activated for all its other functions, such digestion or immune regulation (although we

often assume that is the case).

So if vagus nerve activity is a good thing and HRV is a sign of vagus nerve activity in the heart, is there evidence that higher HRV is a sign of good health?

HRV as a predictor of health

A comprehensive systematic review in 2022, Jarczok et al reviewed studies evaluating HRV as a predictor of all-cause mortality across 38,000 participants followed for an average of 8 years (reference). This review found that lower HRV values predicted higher mortality. Those in the bottom 25th percentile had a 56% higher mortality compared with those in the top 25th percentile. To put this in context, this is a modest correlation; VO2 max, diabetes, and smoking are more strongly correlated with mortality than HRV.

However, it is important to point out that this is correlation not causation. Low HRV correlates with higher age and less fitness, so it may just be a marker for poor health and not a cause of it.

Here’s the key question: If low HRV predicts worse outcomes, does increasing HRV improve them?

Does improving HRV translate to better health?

Here are some interventions that raise your baseline HRV and improve health:

• Aerobic exercise

• Sleep consistency

• Metabolic health

• Minimizing alcohol

Here are some interventions that raise HRV acutely:

• Breathwork

• Cold exposure

These can raise HRV in the moment, but we don’t yet have strong evidence that they improve long-term outcomes.

There was one randomized, controlled trial of HRV-biofeedback in patients with coronary artery disease. Yu et al randomized 210 patients to a control group (no intervention) vs a 6 week training on breathwork. These patients were then followed for 1 year and evaluated for visits to the ER or hospital admissions. The HRV-biofeedback group had a significant reduction in ER visits and hospital admissions. The sample size was too small and follow-up was not long enough to see a difference in mortality. This was a moderate-sized RCT showing large reductions in soft cardiovascular endpoints (readmissions, ER visits) with HRV biofeedback.

What I’m taking away

The vagus nerve is fascinating and deeply connected to how our body regulates stress, digestion, and inflammation. HRV gives us a window into one aspect of the vagus nerve—how it influences the heart. While lower HRV is associated with worse health outcomes, we need to be careful not to overinterpret what that means. HRV is best understood as a marker of physiologic resilience, not a proven target for intervention.

We can increase HRV in the short term with breathing exercises or cold exposure, but we don’t yet have strong evidence that doing so improves long-term outcomes. The interventions that consistently raise baseline HRV—exercise, sleep, good nutrition, and minimizing alcohol—are the same ones we already know improve health and longevity.

So instead of chasing a number on your wearable, focus on the behaviors that improve your physiology. If your HRV rises as a result, that’s a signal you’re moving in the right direction.