Heart & Cardiovascular
The Gaps Between Your Heartbeats
The tiny variation in time between beats hides a bigger story than the beats themselves — it's one of the clearest windows into how well you're recovering.
At a glance
- Heart rate: beats per minute (bpm) — measured by an optical PPG sensor on your wrist.
- HRV: the variation between beats — a window into your nervous-system balance.
- Three readings: Fitaos captures HRV three ways — a glanceable number from the PPG sensor, an ECG-derived view, and nightly continuous tracking with a Lorenz scatter plot.
- Key metrics: SDNN and RMSSD (time domain), LF/HF ratio (frequency domain).
- Why it matters: HRV trends reveal recovery, stress and fatigue — not just a single number.
The Science
Heart Rate & HRV
Your heart rate (HR) is how many times your heart beats per minute. A Fitaos wearable measures it continuously with an optical PPG sensor on the wrist.
But a healthy heart does not beat at perfectly regular intervals. The time between beats varies slightly — this is heart rate variability (HRV). It is not random noise; it is the fingerprint of your autonomic nervous system continuously adjusting your heart. The sympathetic branch (action, stress) speeds it up; the parasympathetic branch (rest, recovery) slows it down. A flexible, well-recovered system produces higher HRV.
Why we research this
Recovery is invisible until it's gone. HRV is the earliest honest signal of it — the same reason every serious recovery brand is built on it. Fitaos treats it as a personal trend, not a score to chase.
The key word is personal. HRV varies enormously between people; your baseline is yours alone. That is why Fitaos anchors everything to your own trend, never to a population average.
Data Sources
Three Ways Fitaos Captures HRV
HRV is not a single measurement — it is a family of readings taken at different moments of your day. Fitaos captures it three ways, each serving a different purpose.
Glanceable HRV (PPG)
On devices with a screen, the HRV number you see at a glance is computed from the beat-to-beat (P–P) intervals measured by the optical PPG sensor on your wrist. Each glance is a single-measurement snapshot that the app logs into your daily HRV trend.
ECG-Derived HRV
When you take an ECG reading, the same capture yields beat-to-beat intervals from the ECG waveform. Taken at a calm, still moment, this adds a second HRV perspective that — together with your daily and nightly trends — helps build a fuller picture of your heart-rhythm awareness.
Nightly Continuous HRV
While you sleep, the wearable tracks HRV minute by minute. Beyond the minute-level series, Fitaos renders your nightly HRV as a Lorenz scatter plot (Poincaré plot) so you can compare nights visually and read your cardiac rhythm at a glance.
The Lorenz (Poincaré) scatter plot turns thousands of overnight beats into one picture: each beat interval is plotted against the next. A tight, elongated cloud means your intervals vary in a healthy, adaptive rhythm; a rounder, more scattered cloud suggests reduced variability. You don't need to be a physiologist to read it — put two nights side by side and the change is obvious.
Important — general wellness informationHRV features provide general wellness information. They are not intended to diagnose, treat, cure or prevent any disease. Use them as personal trend tools, and consult your physician for medical advice.
The Metrics
Reading HRV Numbers
| Metric | Domain | What it reflects |
|---|---|---|
| SDNN | Time | Standard deviation of all beat-to-beat intervals — overall HRV across a full recording. |
| RMSSD | Time | Short-term, beat-to-beat variation — primarily reflects parasympathetic (rest-and-digest) activity. |
| pNN50 | Time | Percentage of successive intervals differing by more than 50 ms — another parasympathetic marker. |
| LF (low frequency) | Frequency | 0.04–0.15 Hz band — reflects both sympathetic and parasympathetic influence. |
| HF (high frequency) | Frequency | 0.15–0.4 Hz band — linked to breathing and parasympathetic (vagal) activity. |
| LF/HF ratio | Frequency | Balance between sympathetic and parasympathetic activity — higher means more stress-dominant. |
For most people, the simplest and most useful approach is to watch your own HRV trend rather than any single number.
Applications
What HRV Powers
🏋️ Training Recovery
Athletes watch HRV trends to decide when to train hard and when to take a lighter recovery day.
💼 Work-Life Awareness
Office workers use HRV trends to notice when stress is building up before burnout.
🛌 Sleep Optimization
Connecting poor sleep nights to next-day HRV helps you prioritize rest.
🧘 Stress Management
HRV is the backbone of stress scores — when stress rises, HRV falls.
Important — general wellness informationHeart rate and HRV features provide general wellness information. HRV values vary widely between individuals and are not intended to diagnose, treat, cure or prevent any disease. Use them as personal trend tools, and consult your physician for medical advice.
How the Leaders Track Recovery
HRV and Energy — The Brand Playbook
Recovery is the most crowded wellness category. Every brand uses heart-rate variability — but packages it differently.
Garmin Body Battery — the best-known energy score: it combines HRV, sleep, stress and activity into a single 0–100 "battery" you watch rise and fall through the day.
WHOOP — Recovery score built from HRV, resting heart rate and sleep performance; a daily "how ready are you to take on strain" read.
Oura — Readiness score from HRV balance, sleep and activity, framed as long-term balance rather than a daily grade.
Apple Watch — samples HRV and nudges mindfulness; no single recovery score, but HRV trend is visible in Health.
Samsung & Fitbit / Huawei — derive HRV-based stress and recovery insights from the same signal.
Fitaos — shows the underlying HRV metrics (SDNN, RMSSD, LF/HF) directly, so you see the signals behind your recovery, not just a number.
Where some brands stop at a single recovery score, Fitaos shows you the signals behind it — so you understand why your recovery is up or down, not just that it is. The principle is the same one WHOOP and Oura are built on.
In the GBAND App
Recovery, Made Actionable
🔋 HRV Trend
GBAND plots your daily SDNN / RMSSD so you can read recovery at a glance.
Nightly HRV & Lorenz Plot
GBAND shows your overnight HRV minute by minute and renders it as a Lorenz scatter plot, so you can compare nights and see your rhythm steady or shift.
🤖 AI Recovery Tip
Daily AI analysis suggests what your low or high HRV trend means for the day ahead.
👨👩👧 Family Sharing
Watch a loved one's HRV recovery trend and notice sustained stress before it becomes a problem.
📊 Long-Term Log
Track HRV over weeks to see real training or lifestyle effects.
Important — wellness informationHRV features are general wellness information. They are not intended to diagnose, treat, cure or prevent any disease. Consult your physician for medical advice.
Common Questions
FAQ
What is heart rate variability (HRV)?
HRV is the variation in time between consecutive heartbeats. A healthy heart does not beat like a metronome — the interval changes slightly with each beat, and this variation reflects your nervous system's balance.
What do SDNN and RMSSD mean?
SDNN is the standard deviation of all beat-to-beat intervals, reflecting overall HRV. RMSSD measures short-term, beat-to-beat variation and mainly reflects parasympathetic (rest-and-digest) activity.
What does the LF/HF ratio tell you?
The LF/HF ratio compares low-frequency and high-frequency components of HRV. A higher ratio suggests sympathetic (stress) dominance, while a lower ratio suggests parasympathetic (rest) dominance.
What is a good HRV?
HRV is highly personal. There is no universal "good" number — what matters is your own trend over time. Generally, higher HRV relative to your baseline indicates better recovery, while a sustained drop suggests stress or fatigue.
How is the HRV number on my watch calculated?
On screen devices, the HRV value you see at a glance is computed from the beat-to-beat (P–P) intervals measured by the optical PPG sensor. Each reading is a single-measurement snapshot that the app logs into your daily HRV trend.
Does an ECG reading also give HRV?
Yes. When you take an ECG, the same capture yields beat-to-beat intervals from the ECG waveform. Because an ECG spot-check happens at a calm, still moment, it adds a second HRV perspective that complements your continuous PPG readings as part of your overall heart-rhythm awareness.
What is a Lorenz (Poincaré) scatter plot?
It plots each heartbeat interval against the next, turning overnight HRV into one picture. A tight, elongated cloud suggests healthy, adaptive variability; a rounder, more scattered cloud suggests reduced variability. Comparing two nights side by side helps you visually track how your cardiac rhythm steadies or shifts.
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Trademark notice. Apple, Apple Watch, Garmin, WHOOP, Oura, Samsung, Huawei, Fitbit and other brand names are trademarks of their respective owners. References to other products are for general-information comparison only and do not imply endorsement, sponsorship, or affiliation.
Feature comparisons reflect generally available consumer capabilities for illustration only; they are not medical claims and do not imply endorsement of or by any brand. Fitaos features are wellness tools, not medical devices.