Your overnight HRV is 45. Your training partner's is 90. Neither of you has a problem - and why that's true is most of what's worth knowing about heart rate variability.
What it measures
Your heart doesn't beat like a metronome. The gap between consecutive beats varies all the time, and HRV measures that variation.
Despite the name, more variability is the healthy signal. It means your nervous system is responsive, switching smoothly between its stress and recovery modes rather than stuck in one gear.
HRV falls first when your body is under load. Poor sleep, alcohol, illness, hard training, dehydration and emotional stress all pull it down, often before you feel any of them. A sustained drop across a week or more is one of the earliest signs of overtraining or an illness brewing, while a steady rise tracks with better sleep, smarter training and real recovery.
Why we use the overnight figure
When NOYU says "HRV" without qualifying it, we mean your overnight figure, measured during sleep.
HRV is sensitive to what you're doing when your device takes the reading. Standing up, drinking a coffee or holding a conversation each move it. A reading from your desk after a coffee and a reading from lying in bed aren't comparable, so a series built from whenever you happened to check tells you little about your trend.
Sleep fixes that. You're lying down, at about the same time of night, doing about the same thing, so night-to-night changes in the sleep window reflect your physiology rather than your posture.
Daily HRV
Some devices also report an all-day HRV figure, built from readings taken through the day as well as overnight. Where we show this, we label it daily HRV so you can tell it apart from the overnight number.
It's a real metric, and a noisier one. Spanning the whole day, it picks up daytime stress, hard sessions and posture as well as overnight recovery, so it can read differently from your overnight HRV on the same date. Some devices don't report it.
We never show HRV measured during a workout as "HRV", because a reading taken mid-exertion carries little recovery information and doesn't compare to either of the above.
Why your number can't be compared to anyone else's
HRV is one of the most individually variable metrics in physiology. Two equally healthy, equally fit people of the same age can differ by a factor of three or more, because genetics, age and anatomy shape it as much as fitness does.
So NOYU never scores your HRV against a population range. What counts is your number and the direction it's moving.
We measure everything we do with HRV against your own 30-day baseline, built from the days before the one being scored.
RMSSD and SDNN
You may see your HRV labelled RMSSD or SDNN. These are two different mathematical ways of summarising the same underlying beat-to-beat variation, and they produce different numbers from the same night.
Which one you get depends on your device. Most report RMSSD, which is the better-validated measure for overnight recovery. Some, including Apple Watch, report SDNN.
NOYU picks whichever your device populates more consistently and uses that one measure throughout - for your current reading, your baseline and your trend. We never mix the two within your history, because a switch mid-series would look like a sudden change in your physiology when nothing about you had changed.
This is also why your NOYU HRV may not match a number you've seen in another app. If that app reports a different measure, the two aren't meant to agree.
How HRV feeds Readiness
HRV is the largest single input to Readiness, and it appears twice:
- HRV against your baseline is the biggest contributor: how last night compares to your own recent normal.
- HRV stability is a smaller one: how much your HRV has bounced around across the last week. Steadiness is a good sign in itself, whatever the level. It needs at least four nights of data before it means anything, so it stays out of your score until then.
Alongside those sit last night's sleep score and your sleeping heart rate against baseline, plus body temperature, respiratory rate and blood oxygen when your device reports them. We drop anything your device can't report and reweight the remaining parts to fill the gap, so a missing piece never costs you.
What to do with it
Watch the trend across days rather than any one night.
A single low reading is usually noise, a late meal or a glass of wine. Act on a run of several days below your normal, especially alongside a rising sleeping heart rate, which together suggest you need an easier few days.
Chasing a higher HRV number day to day won't get you anywhere, and reading each morning's number as a verdict on yourself makes sleep worse.