In the last post, we looked at VO₂ max (maximal aerobic capacity) – an important marker used to measure how efficiently your body uses oxygen during intense exercise. The more oxygen your muscles can access, the better fuel they have to keep you moving.
Now, I want to switch our focus from lungs to heart and talk about something I’ve become increasingly curious about: Heart Rate Variability (HRV).
To be honest, I hadn’t heard of HRV until I started using the Whoop app. It kept showing up in my data like an uninvited guest. What does it actually mean? And why does it matter? This article aims to answer some of those questions.
Heart Rate Variability (HRV)
Heart Rate Variability (HRV) is exactly what it sounds like, it measures the variation between each of our heartbeats. Our hearts don’t beat at the same interval (e.g., every 500 ms), instead healthy hearts all have variability in their beats (e.g., 754ms, 828ms, 845ms, etc…).
This variability is a good thing. It means your body is responding and adapting to internal and external demands in real time. Less variability? That can be a red flag.
Here’s a quick visual to illustrate what HRV might look like:
A Window Into the Nervous System
HRV is a direct reflection of how well your autonomic nervous system (ANS) is functioning. This system controls all the things you don’t consciously think about (e.g., breathing, digesting, heartbeat).
The ANS has two branches:
| Sympathetic system | “fight-or-flight” – Prepares your body for action, stress, or danger. |
| Parasympathetic system | “rest-and-digest” – Helps your body relax, recover, and conserve energy. |
High HRV typically means your body is doing a good job switching between these two modes. Responding to stress when needed, then calming down afterward. Low HRV may mean your body is stuck in “go mode” and struggling to recover.
One study (Tiwari et al., 2021) even found that low HRV is linked to higher mortality rates and can help identify risk factors for cardiac disease. Thought that was pretty interesting.
Key Takeway:
| Low HRV | Heartbeat intervals are more constant → your body may be under stress, overtraining, or fighting something off. |
| High HRV | Heartbeat intervals are more variable → your body is in a balanced, adaptive, and well-recovered state. |
Think of HRV as your body’s internal readiness score. When it’s high, your system is flexible and resilient. When it’s low, it’s time to back off or take a closer look at what’s happening (sleep? nutrition? stress? illness?).
So… What’s a Good HRV?
This is where it gets clear as mud.
There’s no universal “good” HRV score. It’s highly individual and influenced by many factors, including age, gender, genetics, training history, and even the time of day you measure it.
There is a comprehensive article diving into the specifics of each worth reading here – HRV Demographics, Part I – Age & Gender.
If you do some research you’ll find that a lot of wearables, like Whoop, have a lot of literature on HRV. They shy away from giving what good looks like, instead opting to share generalized views of their users. I don’t particularly like that because it shows you a rough average of what others are doing, not what it could, or should be.
For example, here is what Whoop shows in terms of HRV values by age:
Real Life: My HRV Journey
I’m a 42-year-old male, and since I started tracking my HRV six months ago, I’ve seen it climb from around 91 to the 96. This doesn’t take into account however that I’ve been working on my fitness for over three years now. It would have been awesome to have tracked this at the very beginning.
Where this can get interesting is if I wake up and see a sudden dip in my HRV, it’s often a clue. Maybe I didn’t sleep well. Maybe I’m about to get sick. Maybe I overtrained the day before. It gives me an early warning system—before my body even starts to feel off.
Similar to VO₂ max I’m fascinated by HRV. Maybe it’s the ambiguity around it? But it’s probably because I think it gives us insight not just into your fitness level, but our resilience, recovery, and readiness to perform – physically and mentally.
Remember, high HRV means your body is adapting well to stress and recovering efficiently. Low HRV, on the other hand, can signal that your system is under strain and not bouncing back as it should. Put simply: when your HRV is high, you can do hard things and recover from them. A sudden drop might also be your body’s way of flagging something deeper, like illness, fatigue, or emotional stress. You want variability in your heartbeat!
Here is what I recommend you take away from this: Track your own baseline and watch the trends. Don’t get too hung up on the number itself. What matters is how it changes and what it reflects about your habits, stress, and recovery.
So if you’re using a wearable like Whoop, Oura, or even an Apple Watch, take a look at your HRV. It might just be telling you more than you think.