HRV (RMSSD): Beat-to-Beat Variation Explained
Estimated from a single camera (PPG) reading — an educational estimate, not a medical measurement. State labels (Flow, Focus, Alarm…) are rough, experimental patterns, not emotion detection.
What HRV is
Heart rate variability (HRV) is the natural variation in time between heartbeats. A healthy heart does not tick like a metronome: intervals lengthen and shorten with breathing, posture and nervous-system activity. RMSSD, the root mean square of successive differences, is one of the most widely used HRV measures and mainly reflects parasympathetic (vagal) activity, the "rest and digest" side of the nervous system.
How this tool calculates it
The tool takes each pair of consecutive RR intervals, squares their difference, averages those squares and takes the square root, giving a value in milliseconds. Values above 100 ms are capped at 100. The tool works on a rolling 15-second window of fingertip video (about 30 frames per second), finds each pulse peak, keeps only beat-to-beat (RR) intervals between 400 and 1,500 ms that are within 22% of the median, and smooths the displayed numbers so they change gradually.
Typical ranges and what affects them
For short daytime readings like this one, resting RMSSD in healthy adults varies widely, often roughly 20–80 ms. Age and time of day matter: it tends to be higher in younger and fitter people, lower with age, and night-time values from a watch are usually higher than daytime spot checks. See normal HRV by age or try the HRV tool. It usually drops after poor sleep, alcohol, illness, intense training or acute stress, and rises with good recovery. Differences between people are large, so your own trend matters more than comparison with others.
Higher than your usual
Often seen when rested and relaxed.
Lower than your usual
Common with fatigue, stress, alcohol, illness or a late heavy meal.
Why camera readings are rough
RMSSD depends on millisecond-level timing. A phone camera at about 30 frames per second, a fingertip that shifts slightly, and a window of only 15 seconds all add error, usually inflating the value. Standard short-term HRV measurements use at least a few minutes of ECG or a chest strap. Treat the number here as a rough estimate and compare readings only when taken the same way.
Practical tips
Measure consistently
For trends, measure at the same time (for example after waking), sitting still, breathing normally.
Slow breathing
Slow breathing at around 6 breaths per minute typically increases beat-to-beat variation while you do it.
Basics first
Regular sleep, limited alcohol and aerobic exercise are associated with higher resting HRV over time.
The ONDA app is a separate product: it focuses on pulse, breathing and, with an Apple Watch, coherence. It does not use the formulas on this page.
The ONDA Principle
RMSSD reflects vagal activity; the camera version here is a rough estimate, most useful when compared with your own readings.
Ready to measure?
The ONDA app works with your iPhone camera, Apple Watch and Apple Health.