Your heart does not beat like a metronome. Even at 60 beats a minute, the gap between two beats might be 0.95 seconds, then 1.05, then 0.98. That tiny variation is called heart rate variability, or HRV, and it's one of the most useful numbers for understanding how rested you are.
What HRV measures
HRV is the variation in time between heartbeats. It is shaped mostly by your autonomic nervous system, the automatic part that runs in the background:
- The "rest and digest" side (parasympathetic, largely through the vagus nerve) slows the heart a little on each out-breath and lets the rhythm vary more.
- The "fight or flight" side (sympathetic) speeds the heart up and makes the rhythm more even.
So, very broadly, a relaxed, recovered body tends to show more variation, and a stressed, tired or unwell body tends to show less.
The numbers: RMSSD and SDNN
There are many ways to calculate HRV. Two are common in apps:
- RMSSD (root mean square of successive differences): looks at beat-to-beat changes. It is the most used number for short readings and mainly reflects the "rest and digest" side.
- SDNN (standard deviation of the beat intervals): looks at overall spread. It is most meaningful over longer recordings.
Both are shown in milliseconds (ms). Qira shows RMSSD as your main HRV number and SDNN in the details.
What is a "good" HRV?
This is the most asked question, and the honest answer is: compare yourself with yourself. HRV varies a lot between people. It tends to be lower with age, and it depends on genetics, fitness, time of day and even how you breathe during the reading. Two healthy people of the same age can have very different numbers.
What matters is your own baseline and your trend. A reading clearly below your usual range, especially several days in a row, is a hint to go easier. A rising trend over weeks often goes with better sleep, fitness and recovery.
What lowers HRV for a day or two
- Short or poor sleep
- Alcohol, especially in the evening
- A hard workout (normal, as part of training)
- Stress, worry, travel and jet lag
- Getting sick
- A large late meal
What tends to support it over time
- Regular, enough sleep at consistent times
- Regular moderate exercise, with rest days
- Slow breathing: around six breaths a minute, with a longer exhale, is a well-studied way to encourage the rest-and-digest response
- Less alcohol, and earlier, lighter dinners
- Time outdoors and time with people you like
How to take a reliable reading
- Measure at the same time each day, ideally in the morning before coffee.
- Sit still and breathe normally for a minute first.
- Use steady light and keep still during the scan.
- Look at your 7-day trend, not a single number.
HRV from a phone camera
Chest straps and smartwatches measure HRV from electrical or optical signals. A phone camera can do something similar: each heartbeat changes your skin color very slightly, and the camera can track it in your face or fingertip (a method called rPPG). Qira uses this to estimate heart rate, HRV, breathing rate and a stress index in about 30 seconds, and the video is processed on your phone.
Camera readings are estimates and work best in good light while you keep still. They are great for trends and daily reflection, not for medical decisions.
Sources
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation, 1996;93(5):1043–1065.
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017;5:258.
- Wang W, den Brinker AC, Stuijk S, de Haan G. Algorithmic principles of remote PPG. IEEE Transactions on Biomedical Engineering, 2017;64(7):1479–1491.
