Cardivo guides

HRV explained: RMSSD, SDNN, and what's “good”

Updated October 17, 2026 · Wellness education, not medical advice

Key takeaways

HRV is the variation in time between heartbeats, measured in milliseconds.
Higher HRV compared with your own baseline usually signals good recovery; a drop often follows stress, poor sleep, alcohol, illness, or hard training.
RMSSD reflects rest-and-digest (vagal) activity; SDNN reflects overall variability.
HRV is highly individual and falls with age — compare yourself to yourself, at the same time each day.

Your heart doesn't beat like a metronome. Even at 60 bpm, the gap between two beats might be 980 milliseconds, then 1,030, then 1,000. That beat-to-beat variation is heart rate variability (HRV). It reflects the push and pull between the two branches of your autonomic nervous system: the "fight or flight" sympathetic side and the "rest and digest" parasympathetic side.

The main HRV metrics

Metric	What it measures	What it tends to reflect
Mean RR	Average time between beats (ms)	The inverse of heart rate — longer RR means a slower pulse
RMSSD	How much each beat-to-beat interval differs from the next	Parasympathetic (vagal) "rest and digest" activity; the most common short-term HRV metric
SDNN	Standard deviation of all beat intervals	Overall variability; more meaningful over longer recordings
AMo50	Share of beat intervals clustered around the most common value	Higher values suggest a more rigid, stress-dominated rhythm
HRV score	Cardivo's simplified 0–100 summary	An at-a-glance read compared with your typical range

What is a good HRV?

There's no universal good number. HRV varies widely between healthy people and naturally declines with age. Genetics, fitness, breathing, and even the length of the recording change it. That's why researchers and coaches focus on your personal baseline: a reading well below your usual range is more meaningful than comparing your RMSSD with a friend's.

Things that often lower HRV: stress, short or poor sleep, alcohol, illness, dehydration, and hard training without recovery. Things that often support it over time: regular aerobic exercise, consistent sleep, slow breathing, and managing stress.

How to measure HRV consistently

Measure at the same time each day — first thing in the morning works well.
Sit or lie down, breathe normally, and stay still and quiet.
Look at weekly trends rather than reacting to one reading.

Measuring HRV with your iPhone

Apple Watch records HRV as SDNN in Apple Health. Without a watch, Cardivo measures HRV from your fingertip on the iPhone camera in about 30 seconds and reports HRV score, Mean RR, RMSSD, SDNN, and AMo50 — each with one plain-English sentence and your typical range. Short recordings are best suited to RMSSD-style metrics; Cardivo treats every value as a wellness trend, not a diagnosis.

When HRV isn't the right tool

HRV can't diagnose heart rhythm problems such as atrial fibrillation. If you feel palpitations, fainting, chest pain, or shortness of breath, see a doctor — and call 911 in an emergency.

This guide is general wellness education based on the public sources below. It is not medical advice, diagnosis, or treatment. Cardivo is not a medical device. If you think you are having a medical emergency, call 911.

Frequently asked questions

What is a normal HRV?

There's no single normal value. HRV differs a lot between people and drops with age. The most useful comparison is with your own baseline, measured the same way at the same time of day.

Is higher HRV better?

Generally, higher HRV relative to your own baseline suggests better recovery and parasympathetic activity. A sudden drop often follows stress, poor sleep, alcohol, or illness.

What's the difference between RMSSD and SDNN?

RMSSD looks at differences between consecutive heartbeats and mainly reflects rest-and-digest (vagal) activity. SDNN is the overall spread of all beat intervals and is more meaningful over longer recordings. Apple Health reports SDNN.

Can I measure HRV without an Apple Watch?

Yes. Cardivo measures HRV from your fingertip on the iPhone camera in about 30 seconds, reporting RMSSD, SDNN, Mean RR, AMo50, and an HRV score.

Sources

Cardivo is an ad-free iPhone app that reads heart rate and HRV from your fingertip on the camera in about 30 seconds — no Apple Watch needed — and keeps blood pressure, blood sugar, and mood in one journal you can share with your doctor as a PDF. Download on the App Store.