What Is Heart Rate Variability, and What Does Your Number Actually Mean?

If your heart rate is sixty beats per minute, it is almost certainly not beating once every second.

It might be 0.94 seconds, then 1.08, then 0.97, then 1.11. Averaged across a minute, sixty. Beat to beat, constantly varying.

That variation is heart rate variability, and the counter-intuitive part is that more of it is better. A heart that beats with metronome regularity isn’t a well-regulated heart. It’s a heart whose regulatory system has stopped adjusting — and in clinical contexts, very low variability is a marker of poor outcomes rather than good ones.

Why the variation exists

Your heart has its own pacemaker cells, which left alone would fire at around a hundred beats a minute. Your resting rate is lower because your vagus nerve is applying a continuous brake.

That brake is dynamic. It eases fractionally as you breathe in and firms up as you breathe out, which produces the rhythmic rise and fall in heart rate across each breath cycle — respiratory sinus arrhythmia. Layered on top are slower oscillations from your blood pressure regulation system and other control loops.

So HRV is essentially a readout of how actively your regulatory systems are adjusting. High variability means the brake is responsive and your system is making constant fine corrections. Low variability means either the brake is weak, or something is overriding it — usually sympathetic activation.

Which is why HRV has become the most accessible non-invasive marker of autonomic function we have. It’s not measuring your stress directly. It’s measuring the flexibility of the system that manages your stress.

The numbers, and why yours can’t be compared to anyone else’s

The measure most consumer devices report is RMSSD, which reflects the beat-to-beat differences and is largely driven by vagal activity. It’s usually expressed in milliseconds. You may also see SDNN, which captures more of the overall variability including slower influences.

Typical RMSSD values in adults range from around twenty to well over a hundred milliseconds. That’s an enormous spread, and it’s normal.

Here’s what determines where you sit, before anything you do about it:

Age. HRV declines steadily across adulthood. A woman of fifty with a lower number than a woman of thirty isn’t necessarily less well regulated.

Genetics. A substantial portion of the variation between individuals is inherited.

Fitness. Endurance-trained people tend to have higher HRV.

Measurement method. Chest straps, wrist devices and finger sensors produce different numbers, and different apps apply different algorithms to the same raw data. Two devices on the same person will disagree.

Which leads to the single most important thing about HRV: your number is only meaningful compared to your own baseline. Comparing it to a friend’s, or to a chart of population averages, tells you almost nothing useful and generates a fair amount of unnecessary worry.

What matters is your trend over weeks, and your deviation from your own normal.

What your watch is and isn’t telling you

Consumer wearables have made HRV widely available, which is broadly good and comes with some caveats.

What they do reasonably well: overnight or early-morning readings, taken at rest, in consistent conditions. Trends over weeks. Flagging days when you’re notably below your own baseline.

What they do less well: single readings taken at random points in the day. Spot checks during activity. Comparisons against other people. Precision at the level of small daily differences.

A drop of five milliseconds from yesterday means very little. A drop sustained across ten days means something.

And a genuine caution: HRV data can become its own stressor. I’ve worked with several women who check the number first thing, see it’s low, and spend the day interpreting everything through that lens — which isn’t a neutral act, and which quite plausibly affects the next day’s reading. If tracking is making you anxious, the tracking has stopped being useful. The number is meant to inform you, not to grade you.

What pushes it down

Most of these are unsurprising, which is part of why HRV is useful — it aggregates things you already half-know.

Poor or short sleep, and especially disrupted sleep. Alcohol, which affects HRV substantially and for longer than most people expect — often two nights, not one. Illness, frequently a day or two before symptoms appear. Hard training, appropriately, as part of a normal training response. Sustained psychological stress. Dehydration. Late heavy meals. And chronic pain.

Cortisol sits underneath a lot of this. It’s the slow-acting hormone released across hours rather than seconds, and it sets your baseline reactivity rather than producing individual surges. Sustained elevated cortisol is associated with reduced HRV, which is why a stressful month shows up in the data even on days when nothing in particular happened. [LINK: Cortisol and Perimenopause] covers that relationship in more depth.

The perimenopause drop, and why it isn’t your fault

This is the piece that matters most for women in their forties and fifties, and it’s rarely explained.

Oestrogen has a direct supportive relationship with vagal tone and with the responsiveness of your blood vessels. As oestrogen becomes erratic through perimenopause and then declines, HRV tends to fall — measurably, and for reasons entirely outside your control.

Several other things compound it. Sleep fragments, often severely, and sleep is one of the strongest influences on HRV. Night sweats produce repeated autonomic arousals you may not fully wake for. Palpitations become more common. And progesterone, which affects both respiratory drive and — through its metabolite allopregnanolone — the calming systems in the brain, becomes unpredictable.

So a woman who has tracked her HRV happily for years may watch it decline through her late forties while doing everything the same. That’s frequently interpreted as failure. It isn’t. It’s a physiological transition showing up in the data exactly as you’d expect.

What’s more useful in this stage is watching your own trend within the new range, and noticing what moves it. The absolute number will be lower than it was at thirty-five. That’s not the interesting question.

What actually raises it

The evidence here is uneven, and I’d rather be honest about the hierarchy.

Strongest and most direct: slow breathing. Breathing at around five to six breaths a minute produces large, immediate increases in HRV — this is one of the most reliable findings in the field. Done regularly, it appears to improve baroreflex sensitivity, which is the underlying mechanism. Ten minutes daily is a reasonable dose.

Strong: sleep. Not glamorous, and more effective than almost anything else. Protecting sleep does more for HRV than any supplement.

Strong: regular aerobic exercise, at moderate intensity, consistently. Note that hard training temporarily lowers HRV — that’s the training stimulus, not a problem, provided recovery follows.

Moderate: reducing alcohol. Often the single most visible change in someone’s data, and frequently the least welcome piece of information.

Weaker or overstated: cold exposure, gargling, humming and singing all appear regularly on lists of vagal exercises. Cold water on the face does trigger a genuine reflex that slows the heart acutely. Whether these produce lasting increases in resting HRV is much less established than the internet suggests. They’re not useless. They’re just not the main lever.

The main lever is breathing, and specifically the everyday pattern rather than the ten-minute practice. If your habitual breathing is fast, shallow and upper-chest, you’re supplying a mild sympathetic input every few seconds all day, and no amount of morning practice offsets that. The Breathing Quiz shows you what your breathing pattern is doing, which is one of the most reliable early signals of how your nervous system is coping underneath.

A sensible way to use the number

Take readings at a consistent time — first thing in the morning is standard, before you look at your phone.

Look at seven-day averages rather than daily figures.

Treat a sustained drop as a prompt to look at your inputs: sleep, alcohol, training load, workload, illness. It’s a question, not a verdict.

Don’t chase it. Doing more in order to improve the number frequently backfires, because effort and vigilance are themselves sympathetic inputs.

And if the tracking is generating more anxiety than insight, stop for a month. The absence of data will tell you something too.

Where to take this next

If you want your body to learn the slow, resonant rhythm that raises HRV — without counting or a timer — The Reset Audios guide the pace for you. The body learns the frequency by being taken there repeatedly, and that’s what shifts a baseline rather than a single morning’s reading.

If you’d rather understand the mechanism and build the practice properly, The 30 Day Breath Reset takes you through the science and the daily structure together.

Used together, you get both sides of the change — the understanding and the experience. That’s what holds.

And if your HRV has dropped through perimenopause alongside fragmented sleep, palpitations and a stress response that’s stopped behaving the way it used to, that’s a layered picture that a general programme won’t fully reach. Working with me one to one means we look at your specific pattern and stage rather than at population averages.

Related reading: Cortisol and Perimenopause |  What is Breathwork