Hi Friend

I posted this week about how I use heart rate variability with patients. A lot of you asked what it actually is. So here it is, properly.

I want to start with the thing that surprises people most.

A healthy heart is not regular. If your heart beat at exactly 60 beats per minute with exactly one second between every beat, that would be a bad sign, not a good one. Healthy hearts are constantly making tiny adjustments, speeding up fractionally on the in breath, slowing on the out breath, responding continuously to what your body needs.

Heart rate variability is the measurement of those adjustments. And for the conditions I treat, it is one of the more useful numbers we have.

What it is actually measuring

Not your heart. Your nervous system.

Your autonomic nervous system runs everything you do not consciously control, and it has two branches pulling in opposite directions. The sympathetic branch is the accelerator, the fight or flight side. The parasympathetic branch is the brake, the rest and digest side, working mainly through the vagus nerve.

When the brake is working well, your heart rate fluctuates freely from beat to beat and your HRV is higher. When your system is stuck with the accelerator down, those fluctuations flatten out and HRV falls.

So a low HRV is not really telling you something about your heart. It is telling you your nervous system is braced. Which, if you live with POTS, ME/CFS, Long Covid or fibromyalgia, you probably did not need a wearable to suspect.

The number you are looking at

Your device will show you one of two things.

RMSSD measures the size of the small changes between one beat and the next. It reflects vagal activity fairly directly and it is the number most consumer wearables report. This is the one to pay attention to.

SDNN measures overall spread across the whole recording. It is influenced by both branches and by the length of the recording, so it is less clean but appears on some devices.

If your app just says "HRV" with no explanation, it is almost certainly RMSSD.

Now the part that matters most

Do not compare your number to anyone else's. Ever.

HRV varies enormously between individuals for reasons that have nothing to do with health. It falls with age, differs between people of the same age and fitness, and shifts with hydration, alcohol, position, breathing rate, sleep and where you are in your menstrual cycle. A perfectly healthy forty year old might sit at 25. Another might sit at 90.

There is no good number. There is only your number, and the direction it is moving.

This matters because I have had patients genuinely distressed by an HRV of 18, comparing themselves to a forum post from a marathon runner. That comparison is meaningless. What is meaningful is whether your 18 was a 24 last month.

What the research shows in our conditions

The pattern is remarkably consistent across all of them: reduced vagal activity, a nervous system running with the handbrake off.

In fibromyalgia, lower RMSSD tracks with greater pain intensity, worse fatigue and poorer sleep quality.

In ME/CFS, resting variability is reduced, but the more telling finding is what happens under challenge. HRV drops more than expected on standing, and takes longer than expected to recover afterwards. That is post exertional malaise showing up as a measurement rather than a description, which is quietly significant for a condition that has spent decades being told it is not real.

In Long Covid, the picture looks similar, with persistently reduced vagal activity and symptoms of tachycardia, palpitations and orthostatic intolerance that line up with it. Lower HRV correlates with worse fatigue and cognitive symptoms.

I will add one honest caveat. Most of this is correlation, from studies with modest numbers. HRV is not diagnostic of any of these conditions and a normal reading does not rule anything out. It is a useful signal, not a verdict.

What you can actually do with it

Three things, in order of usefulness.

Spot your crashes before they arrive. This is the main clinical value. A sustained drop over several days often precedes a flare, which gives you a window to pull back before the crash rather than after it. Patients who learn to read this get better at pacing than patients working on symptoms alone, because symptoms lag and HRV does not.

Test what actually helps you. You are going to try things. HRV gives you something objective to check them against over weeks, rather than relying on memory, which is unreliable when you are unwell.

Prove to yourself that pacing works. This is underrated. Watching the number recover after a genuinely restful few days is, for a lot of people, the first tangible evidence that resting is doing something.

One thing I will not tell you to do is push through. The evidence for graded exercise in ME/CFS was reviewed and it was removed from NICE guidance in 2021 for good reason. Pacing within your limits is the approach with support behind it, and HRV is a tool for finding those limits rather than for beating them.

Everything above assumes your number means something. Often it does not, because of how it was measured.

Continuous overnight readings from a wearable are noisier than most people realise, and I think spot checking beats them for the purpose most of you actually care about. Tomorrow I will explain why, exactly how to do it, and how to read the trend without driving yourself mad over daily fluctuations.

If you have started tracking and your number confuses you, reply and tell me. I will use the common ones next week.

Regards

Dr Ahmed