Your heart doesn’t beat like a metronome. The interval between each beat varies slightly — by tens of milliseconds, beat to beat. The interval shortens slightly when you inhale, lengthens slightly when you exhale, fluctuates with emotion, attention, posture, and effort.
That variation is heart rate variability. HRV. The number most wearables now surface as a daily score. HRV Breathe (by SUUR) is built around it — we read it from Apple Health before and after every session so you can see whether the practice is actually working.
Why a varying heartbeat is a good thing
Counterintuitively, a steady heart rate is a sick heart rate. A completely regular pulse — every beat the same distance from the last — is what you see in patients with severe heart failure, advanced diabetes, or extreme stress. A healthy heart adapts continuously to what the body needs. Bigger swings between beats reflect a more responsive autonomic nervous system.
The autonomic nervous system has two opposing branches. Sympathetic is the gas pedal — fight, flight, focus, urgency. Parasympathetic is the brake — rest, digest, recover, restore. The balance between them shifts constantly. HRV reflects how quickly that balance can shift.
SDNN and RMSSD — which one is on your screen?
There are several ways to compute HRV from a stream of heartbeats. Two are common in consumer devices:
- SDNN — Standard Deviation of Normal-to-Normal intervals. The basic statistical spread of the beat intervals. Apple Watch reports SDNN, and HRV Breathe reads it from Apple Health.
- RMSSD — Root Mean Square of Successive Differences. Better captures the short-term variation driven by the vagus nerve. Oura, Whoop, and most overnight HRV measurements use RMSSD.
The two metrics correlate, but their absolute values differ — which is why your Apple Watch HRV number is different from your Oura ring number. Trends matter more than absolute values. Compare yourself to yourself.
What lowers your HRV
- Acute and chronic stress
- Poor sleep, particularly broken sleep
- Alcohol — even one drink lowers HRV measurably for 24+ hours
- Illness, especially viral infection (often the first signal)
- Overtraining or insufficient recovery
- Dehydration
- Anxiety, particularly anticipatory anxiety
What raises it
- Slow breathing at the body’s resonance frequency (~6 breaths per minute) — the most reliable intervention, and what HRV Breathe is built around
- Aerobic fitness (long-term)
- Consistent, adequate sleep
- Cold exposure (brief)
- Vagus nerve activation broadly — humming, gargling, slow exhalation
- Time outdoors
- Active emotional regulation
What HRV is not
It’s not a measure of cardiovascular fitness (use VO₂ max or resting heart rate). It’s not a measure of stress directly (it’s a measure of autonomic flexibility, which correlates with stress recovery). It’s not a diagnostic tool for medical conditions. And it’s not a number you should chase compulsively — the act of obsessing over your daily HRV will lower it.
Use it as a slow signal. Check the trend weekly, not hourly. Pair it with one intervention you can actually do — slow breathing, sleep, less alcohol — and watch the trend move.