THE BIOLOGICAL CEO BRIEF · EPISODE 2
You wake up and, before you’ve reached the kettle, your watch has delivered its verdict: recovery low, body not ready. Do you cancel the session, push through, or ignore it because you feel fine? The number behind that verdict is heart rate variability (HRV). Here’s what it measures, what moves it, and how to use it without letting it run your day.
Listen to the Episode
About 7 minutes. The full briefing follows below if you’d rather read.
Executive Brief
- What it is: the variation in time between heartbeats, a reading of how much spare capacity your system has this morning. Not how fit you are, but how ready you are.
- The key rule: compare it only with your own trend, never with someone else’s number.
- The biggest lever: alcohol. Even a low intake measurably reduced overnight recovery in a study of more than 4,000 working adults.
What HRV Actually Is
Start with a counter-intuitive fact: a healthy heart is not a metronome.[1] Even at rest, the gap between one beat and the next keeps changing, a few milliseconds longer, then a few shorter. Heart rate variability is simply a measure of that variation.
It matters because the variation is controlled by your autonomic nervous system, the part of you that runs without being asked. It has two sides. One prepares you for action: stress, effort, deadlines. The other handles rest, digestion and repair.
When the rest-and-repair side is in charge, your heart responds quickly to every breath and the gaps between beats vary more. Higher HRV therefore generally signals a system that is calm, adaptable and recovering. Under strain, the beats become more regular and HRV drops.
Think of it as a reading of how much spare capacity your system has this morning. Not how fit you are. How ready you are.
Why Your Number Is Personal
The most common mistake is comparing your number with someone else’s. HRV varies enormously between people. It falls naturally with age, differs between men and women, and depends on how and when it’s measured, which is why one device’s score isn’t directly comparable with another’s.[1]
So a 45-year-old with an HRV of 35 is not necessarily less healthy than a 30-year-old at 70. Researchers who have reviewed the published norms make exactly this point: context changes the baseline.
Establishing your baseline: measure the same way every day (most wearables use your overnight or first-morning reading) and watch your normal range over two to four weeks. The signal is in the trend, and in sharp departures from it, not in any single morning.
What Pushes It Down
Several things move the number, and most of them are in your control:
- Poor or short sleep: the most common cause. See Recovery Science & Sleep Architecture.
- A heavy meal late at night.
- Illness: often before you feel any symptoms.
- Hard training: in the day or two afterwards.
- Sustained psychological stress: the board meeting, the deal, the 11pm emails. See How to Reduce Cortisol Naturally.
The Clearest Effect: Alcohol
A Finnish study tracked 4,098 working adults (average age 45) through their normal lives, comparing each person’s nights with alcohol against their own nights without.[2] The effect was dose-dependent. Even a low intake measurably reduced recovery during the first three hours of sleep. On the study’s HRV-derived recovery measure, low, moderate and high intake reduced recovery by around 9, 24 and 39 percentage units respectively.
Two details are worth sitting with. Being physically active didn’t protect people from the effect. And it showed up at low doses, not just heavy ones. That one glass of wine with dinner? Your nervous system notices, even if you don’t.
How to Use It
- A nudge, not a verdict. A low reading is a question, not an instruction. Did you sleep badly, drink, eat late or train hard? Usually the answer is obvious, and that is the value: it makes cause and effect visible.
- Adjust the intensity, not the habit. If your HRV is well below your normal range, swap the hard session for an easy one: Zone 2, a walk or mobility work. Don’t skip movement altogether.
- Look for patterns over weeks. A downward drift is a signal about workload, sleep or stress, long before burnout announces itself.
- Don’t let it make you anxious. If checking your score every morning becomes a source of stress, it’s working against you. Checking weekly instead is perfectly reasonable.
There’s research behind this approach. In a small randomised trial of 26 men, those whose daily training intensity was set by their morning HRV improved their maximal running speed more than a group following a fixed plan, and only the HRV-guided group significantly raised its VO2 peak.[3] It was a small study, so treat it as promising rather than proven. But the principle is sound: train hard when you’re ready, back off when you’re not. For more on raising your aerobic ceiling, see the VO2 Max Guide.
The Takeaway
HRV reflects how ready your system is to perform. Compare it only with your own trend. Watch sleep, late meals, stress and, above all, alcohol. And use it to adjust your day, not to dictate it. Measure what matters, and manage it like the asset it is.
This article is for general information and does not replace personalised medical advice. Speak to your GP or a qualified clinician if you have concerns about your heart health or before making significant changes to your training.
Evidence & References
- Shaffer F, Ginsberg JP. (2017). An overview of heart rate variability metrics and norms. Frontiers in Public Health. doi:10.3389/fpubh.2017.00258
- Pietilä J, et al. (2018). Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep in a large real-world sample of Finnish employees. JMIR Mental Health. doi:10.2196/mental.9519
- Kiviniemi AM, et al. (2007). Endurance training guided individually by daily heart rate variability measurements. European Journal of Applied Physiology. doi:10.1007/s00421-007-0552-2
Podcast music: J.S. Bach, Cello Suite No. 3 in C, BWV 1009, Allemande, performed by János Starker. Public domain via Musopen.
