Crack the Code to Peak Performance: HRV Metrics Explained
Heart Rate Variability (HRV) has become an essential tool for tracking recovery, readiness, and overall health. Whether you're an athlete, a fitness enthusiast, or just looking to optimise your well-being, understanding HRV can help you train smarter and recover better.
But with different HRV metrics floating around, which one actually matters for training and recovery?
If you've seen SDNN and RMSSD in your HRV data and wondered what they mean (or if they’re just fancy numbers to confuse you), this article will break it down in plain English.
Understanding the Autonomic Nervous System and Why It Matters
To understand HRV properly, we need to talk about the autonomic nervous system (ANS). This system controls involuntary bodily functions, including heart rate, digestion, and respiration.
It has two key branches:
Sympathetic Nervous System (SNS) – "Fight or Flight"
Activates in response to stress, physical exertion, or danger.
Increases heart rate, blood pressure, and energy output.
Essential for performance but can lead to overtraining if constantly dominant.
Parasympathetic Nervous System (PNS) – "Rest and Digest"
Promotes relaxation, recovery, and energy conservation.
Lowers heart rate and increases HRV (especially RMSSD values).
Critical for recovery, muscle repair, and overall well-being.
A well-functioning autonomic nervous system allows your body to switch efficiently between these states. If you're constantly in fight-or-flight mode (SNS-dominant), recovery suffers, and you may experience fatigue, poor sleep, and reduced performance. This is why HRV—particularly RMSSD—is an excellent indicator of how well your parasympathetic nervous system is functioning.
What is HRV and Why Does it Matter?
HRV (Heart Rate Variability) measures the variation in time between heartbeats. Unlike a metronome, your heart doesn’t beat in perfect sync—it speeds up and slows down depending on your nervous system’s balance.
A higher HRV generally means better recovery, adaptability, and resilience to stress, while a low HRV may indicate fatigue, overtraining, or high stress.
But HRV isn’t just one number. Two of the most common ways to measure it are SDNN and RMSSD—and each tells a different story.
SDNN vs. RMSSD: What’s the Difference?
Both SDNN and RMSSD measure HRV, but they focus on different aspects of your nervous system.
1. SDNN (Standard Deviation of NN Intervals)
What It Measures:
Overall heart rate variability, considering both sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous system activity.
Typically measured over a longer period (hours to full days).
What It’s Good For:
Assessing long-term stress resilience and autonomic nervous system health
Seeing big-picture trends in HRV over weeks/months
More commonly used in medical and clinical settings
How to Interpret It:
✅ Higher SDNN = Better adaptability to stress, good overall health
❌ Lower SDNN = Potential chronic stress, poor recovery, or long-term fatigue
2. RMSSD (Root Mean Square of Successive Differences)
What It Measures:
Short-term beat-to-beat variations, primarily controlled by your parasympathetic (recovery) nervous system.
More reactive to daily changes in sleep, stress, and training load.
What It’s Good For:
Daily recovery tracking (e.g., should I train hard today or take it easy?)
Readiness scores in apps like Oura, Whoop, and HRV4Training
Understanding how well you’re bouncing back from workouts and stress
How to Interpret It:
✅ Higher RMSSD = Strong parasympathetic activity, good recovery, ready for intense training
❌ Lower RMSSD = Fatigue, high stress, poor recovery—consider a lighter session or rest
Which Wearables Use Which HRV Metric?
Different wearables and fitness apps rely on either SDNN or RMSSD (or a combination) for HRV tracking. Although sometimes hard to know I’ve dug a little deeper and found how some of the most popular ones measure HRV (don’t shoot me if its not 100% correct:
Oura Ring – Uses RMSSD and presents HRV as an overnight average.
Whoop Strap – Uses RMSSD, taken during deep sleep, to assess recovery and readiness.
Apple Watch – Uses SDNN, typically measured in short sessions via the Breathe app. However, certain apps like Athlytic (I use this one and always recommend it to Apple Watch wearers) allow you to select the type of HRV reading used, providing more flexibility in tracking your recovery.
Garmin – Uses SDNN for general HRV tracking but RMSSD for its Body Battery feature.
Polar – Uses RMSSD, primarily for recovery tracking in endurance training.
HRV4Training – Uses RMSSD, often calculated from morning readings.
Knowing which metric your device uses can help you better interpret your HRV trends and apply the data to your training and recovery strategies.
The Limitations of Wearables and the Importance of Listening to Your Body
While wearables provide valuable HRV data, they are not perfect. Factors like sensor quality, close skin contact, skin colour, tattoo’s, movement during measurements, and even hydration levels can impact accuracy. Some devices take HRV readings at different times of the day, which can lead to inconsistent results too. Rather than relying solely on numbers, it’s crucial to pair HRV insights with how you actually feel. If your wearable says you’re fully recovered but you’re feeling sluggish, sore, or mentally drained, don’t ignore those signs. Metrics are useful guides, but your body’s signals should always take priority in deciding whether to push hard or take a rest day.
How Can You Improve Your HRV?
If your HRV is consistently low, don’t panic—think of it as your body flashing a yellow warning light. Instead of ignoring it, take action with these practical strategies to improve recovery, boost resilience, and optimise training.
✔ Prioritise sleep – This is non-negotiable. Aim for 7-9 hours per night, keep a consistent schedule, and ditch the late-night scrolling. If your sleep is off, your HRV will be too.
✔ Manage stress – Chronic stress tanks HRV faster than an overcooked steak. Deep breathing (like box breathing), meditation, or even just a 10-minute walk outside can help regulate your nervous system and improve HRV scores over time.
✔ Fuel your body properly – No, you can’t out-train poor nutrition (people know this but oftern forget). Ensure you’re eating enough protein, slow releasing carbs, healthy fats, and micronutrient-rich foods to support recovery. Low energy availability (not eating enough) can crush HRV.
✔ Balance training intensity – Not every session should leave you feeling like you survived a boss fight. Rotate heavy, intense workouts with low-intensity sessions or active recovery days to prevent burnout and improve HRV. You’ll often see HRV drop the day or couple days after a touch session.
✔ Stay hydrated – Dehydration can tank HRV and recovery. Keep water intake steady, and consider adding electrolytes if you’re sweating a lot in training.
✔ Laugh more – Seriously. Studies show that genuine laughter activates the parasympathetic nervous system and boosts HRV. So, watch a comedy, share a joke, and let your nervous system chill out. Stop taking fitness so seriously.
By making small, consistent tweaks, you can help your body shift into a more balanced autonomic state, improving your HRV and overall well-being.
SDNN vs. RMSSD—Which Should You Track?
For most people looking to train smarter, recover better, and avoid burnout, RMSSD is the metric to focus on. It provides real-time insights into how well your body is bouncing back from stress, allowing you to make informed decisions about training intensity, rest days, and overall wellness. By regularly tracking RMSSD, you can fine-tune your recovery strategies and ensure you're progressing without unnecessary setbacks. If your RMSSD is trending low, it’s a sign to prioritise sleep, nutrition, stress management, and lighter training sessions to bring your body back to balance.
Want More Personalised Help?
Tracking HRV is just one piece of the puzzle. If you want a training and nutrition plan built around your recovery and performance, I can help.