- HRV rebounds early in the week (A validation of last week’s finding!) The strongest positive swings in HRV occur on Monday and Tuesday, suggesting that the nervous system tends to recover. Possibly a delayed recovery after weekend rest. Maybe we should all do our hardest training on a Tuesday?
- Negative swings cluster at weekends. Drops in HRV are most frequent on Saturdays and Sundays, likely reflecting cumulative fatigue, social strain, alcohol, or later nights.
- Big HRV changes are rare but meaningful. Only about 4% of daily HRV shifts exceed ±2 standard deviations, but these moments signal significant physiological stress or recovery. Tracking these outliers helps identify when the body is truly under strain or bouncing back.
Sleep Tracking Series
Descriptive HRV using Z scores
Based on an analysis of nearly 100,000 HRV records, this report reveals that most daily HRV changes are small, but larger swings signal significant stress or recovery. The findings show a clear weekly pattern: positive HRV changes (recovery) are most common on Mondays and Tuesdays, while negative changes (fatigue) cluster on weekends.
Alistair BrownleeHead of Research
Cameron CrawfordAI Engineer
September 19, 2025
Key takeaways
Descriptive HRV using Z scores
In the last blog, we looked at the general patterns of HRV and Sleep data for a population of sleep tracker users. We described the population's absolute and delta HRV, as well as sleep data, using the means, which revealed the general pattern. This week, we go a little deeper. Firstly, by standardising the data using Z-scores and secondly, by looking at individual variable links.
The analysis involved nearly 100,000 HRV records from April to June 2025, collected from 1,199 users of wearable devices. Our goal? Understand the magnitude and direction of daily HRV changes and determine if these changes follow meaningful patterns across days of the week.
We used a general HRV metric, which is calculated using several appropriate overnight sample periods using the RMSSD method.
Why Focus on Delta HRV?
While raw HRV can be a helpful snapshot of autonomic recovery, how HRV changes from one day to another is more meaningful. A large drop in HRV might signal poor sleep, late-stage fatigue, or elevated sympathetic stress (or all of the above!) A rebound might reflect recovery, adequate rest, or reduced strain. But — critically — a 10 ms drop means very different things for different people. That’s where z-scores come in. It allows us to “smooth” the data and compare the magnitude of HRV changes.
A z-score lets us measure how far a daily HRV change deviates from a person’s typical variability. Instead of using a fixed threshold, such as “±10 ms”, we use the individual’s mean and standard deviation to define significance. A z-score of +2 is probably a strong signal of recovery. “This HRV increase was two standard deviations larger than usual.” A z-score of minus 2 means “this was an unusually large drop in HRV” — potentially a flag for stress or strain.
Using z-scores allows us to: compare different users on a fair playing field, quantify physiological surprises and track extreme events, not just typical trends. I can hear the statisticians shouting at me at this point, but are delta HRV Z-Scores normally distributed!? The answer is not quite. But we are not using Z-scores here to make strict inferences, just as a way to standardise different days’ scores for comparison.
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Defining HRV Violation Severity
Here’s how we bucket HRV delta z-scores based on their severity:
| Severity | Z-Score Range | Positive frequency % | Negative frequency % |
|---|---|---|---|
| Normal | -1 ≤ z ≤ +1 | 66.25 | 67.7 |
| Mild Violation | 1 < | 19.15 | 19.98 |
| Moderate Violation | 1.5 < | 9.79 | 8.66 |
| Severe Violation | 2 < | 4.64 | 3.58 |
| Extreme Violation | 3 < | 0.16 | 0.07 |
Key Findings
From April 1st to June 24th, 2025, we analysed, 95,623 daily HRV deltas, across 1,199 users.
Z-score overview:
There were 46,365 (48.5%) positive Z-scores, 49,150 (51.4%) negative Z-scores. With 30,584 (32%) violations over Z=1 and 4,210 (4.4%) violations over Z=2. The mean positive Z-score was +0.825, and the mean negative Z-score was -0.778.
Breakdown by Severity
Most HRV swings fall into the mild or moderate category, but severe or extreme drops still occurred in more than 1 in 100 cases — suggesting meaningful day-to-day stressors are not rare, and the possibility of noise based outliers.
Weekly Patterns: When Swings Happen
You can see from figure 1 that large positive HRV changes tend to happen early in the week with negative changes happening more frequently over the weekend.
The implications here are striking and back up the trends we identify in our last blog. HRV tends to rebound early in the week, following weekend rest — but dips sharply again by Sunday, likely reflecting the cumulative load of the workweek or weekend activity.
What Does This Tell Us?
This data suggests a few takeaways for users, coaches, and anyone interested in nervous system recovery:
Your HRV is most likely to rebound on Monday or Tuesday — as we summerised last week, don’t skip the start of the week.
Negative swings are most common on weekends — we are guessing here, but possibly due to overreaching, alcohol, social strain, or late nights!
Big changes are relatively rare — but they matter. The 4% of changes greater than z=2 represent moments of physiological significance.
Delta HRV moves in similar positive and negative jumps. This surprised us. Our personal experience is that the negative jumps tend to be larger than the positives.
Final Thought
Recovery is very dynamic, not static, and often HRV is looked at solely in terms of multi-day trends. We wanted to investigate delta HRV and Z-scores to see if any insights emerged from trying to look at a more dynamic HRV based metric.
In reality this tell us what we already know; our HRV is in constant flux, mostly involving small changes and less frequently involving large changes. The one surprise, for us, that did emerge was that the positive swings are slightly larger, but more concentrated in terms of the days they occur on. The trend is for people to show relatively larger recovery over Sunday and Monday nights followed by a downward trend of relatively smaller negative swings during the week.
If your HRV takes a dive on Sunday — maybe skip the extra set on Monday. If it spikes on Tuesday? Lets go for it!
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Marathon Series
The Data Behind the London Marathon
While Sabastian Sawe was busy breaking the two-hour marathon barrier in London, the watches of 571 amateur runners told a quieter but equally fascinating story. GPS devices over-measured the course by 527 metres on average, faster runners ran with higher heart rates than the slower ones did, and Coros watches reported 47% more calories per kilometre than physiology actually supports.
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Tube Strikes Made Londoners Active
During the April 2026 Tube strikes, Londoners adapted by cycling more—activity tripled, e-bike commutes rose 13%, and 19% of cyclists were entirely new to riding. Data analysis of 1,566 weekday rides revealed e-bike share jumped from 54.9% to 67.5%, with commutes often staying local. The strikes disrupted transit but highlighted Londoners' resilience in finding alternative ways to move.
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April 23, 2026
Heat Exposure
Cold Plunging Might Make Your Biomarkers Worse
We analyzed large-scale wearable data to understand what cold exposure actually does to your body. Sporadic plunges act as a stressor, raising sleeping heart rate. But with consistent use, around 3 sessions every 14 days, the effect flips, improving recovery and sleep scores. We also found that women's responses differ significantly by menstrual cycle phase.
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