How noisy is one morning's reading?
HRV and resting heart rate move from day to day. Four papers show how much, why a 7-day average moves less, and why your own usual range is the yardstick.
By Lomba 4 min read
A tracker gives you a resting heart rate and a heart rate variability (HRV) number every morning. It moves from day to day, even in weeks when your routine stays the same. A 2020 study in PLOS ONE measured how much, for resting heart rate, in 92,457 people.
We read it and three smaller papers. Here is what they measured, what they leave open, and the one idea we think matters most.
What the study did
- Who took part
- 92,457 adults in the United States who wore a wrist-worn tracker for at least 35 weeks between March 2016 and February 2018.
- What was measured
- One resting heart rate per person per day, for a median of 320 days each.
- What was compared
- People with each other, and each person's days with their own other days.
- The other three
- A study of 24 runners, a check of five trackers against a chest strap, and a review of heart rate measures in athletes.
- Who paid
- A US National Institutes of Health grant partly supported two authors of the large study. The maker of the runners' monitors supplied them and part of that study's funding. The US Air Force Research Laboratory funded the tracker check. The review states no funding. None of the four declares a competing interest.
Four takeaways
People differ far more than one person's days do.
Each person's average resting heart rate sat between 40 and 109 beats per minute (bpm), while the changes within one person were "an order of magnitude narrower". For most people, a typical week had 3 bpm between its highest and lowest day. One person in five had at least one week where that gap reached 10 bpm or more.
How it is measured changes the noise.
A 2024 study followed 24 recreational runners through three weeks of regular training. HRV taken with a chest strap, lying down after waking, varied from day to day by 8.2% on the study's log scale, where percentages run smaller than they would in milliseconds. The whole-night figure from a wrist-worn watch varied by 4.2%, and by 1.6% to 8.2% depending on the runner.
The tracker adds error of its own.
A 2025 study had 13 adults wear a chest strap and several of five trackers at once, at home, for 536 nights in all. Nightly HRV from the trackers differed from the chest strap's by 6% to 16% on average, depending on the tracker. Resting heart rate, checked on four of them, differed by 1 to 2 bpm.
Averaging shrinks the noise.
A 2014 review gives the rule: noise falls with the square root of the number of readings, so "using 4 tests halves the noise". It reports that in endurance athletes, links with running performance showed only in averages of at least three to four days, not in single days. The runners' study read its own result from a 7-day average.
The dot is the average across the 24 runners. The line runs from the runner who varied least to the one who varied most.
The same 24 runners over the same three weeks. Point at a row to read how far apart the runners were.
This person’s usual band: the middle half of the 90 single mornings, 56 to 72 ms.
Single mornings. They run from 43 to 101 ms, with a spread of 11.3 ms (a standard deviation). 42 of 90 fall outside the band.
The main takeaway
One morning's number says little by itself. The yardstick that fits is that person's own recent mornings, not a fixed number.
People's own averages sit far apart. In the large study, one person's usual could differ from another's "by as much as 70 bpm".
The study read its own data that way: it counted a stretch as unusual only when readings ran more than 2 standard deviations above the average of that person's own previous 20. A run of mornings says more than any one of them.
What Lomba does with that
A pattern in Lomba never rests on one morning. Lomba lines up the mornings after a habit against all your other mornings, on measures a wearable fills in such as recovery and HRV, and prints the count of mornings behind each side. A first look needs 5 days with the habit and 5 without. Until then the answer is "Still collecting", and no gap is shown.
Chance can make a gap by itself. So Lomba shuffles which mornings count as "after", keeping neighboring days together, and counts how often a gap that large turns up anyway.
It has limits, and we would rather say them:
- Narrow samples. The papers cover tracker owners, 24 runners, 13 volunteers and a review of athletes. Your own mornings may move more or less than theirs.
- Small gaps hide. With enough days and no clear difference, Lomba says "No clear gap". That isn't proof of no effect.
- Not a cause. What Lomba finds is a pattern in your own days, not proof of cause, and not medical advice. Lomba will never set targets you didn't set, or tell you what a result means medically.
Sources
- Quer G and others. Inter- and intraindividual variability in daily resting heart rate and its associations with age, sex, sleep, BMI, and time of year: Retrospective, longitudinal cohort study of 92,457 adults. PLOS ONE, 5 February 2020.
- Nuuttila OP and others. Morning versus Nocturnal Heart Rate and Heart Rate Variability Responses to Intensified Training in Recreational Runners. Sports Medicine - Open, 6 November 2024.
- Dial MB and others. Validation of nocturnal resting heart rate and heart rate variability in consumer wearables. Physiological Reports, August 2025.
- Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome? Frontiers in Physiology, 2014.
The first three are studies. The fourth is a review by one author.