What Apple Watch measures as HRV at night

Open the Health app on your iPhone, look under "Heart", and you will find a reading called heart rate variability – HRV for short. The number comes from your Apple Watch, and it tends to move more than you would expect from a vital sign: 38 today, 61 tomorrow, 45 the day after. This article explains what the watch actually measures, how often it does so, and why that movement is the normal case rather than a fault.

What heart rate variability actually is

Heart rate variability describes how much the time between two heartbeats varies. A heart never beats like a metronome: the gap between beat one and beat two is slightly different from the gap between beat two and beat three. How much those gaps vary is related, among other things, to the state of your autonomic nervous system – the part that runs your heartbeat, breathing and digestion without you doing anything about it consciously.

How Apple Watch records HRV

Apple Watch has no dedicated HRV sensor. It uses the same optical heart sensor on your wrist that already tracks your pulse throughout the day. From a sequence of individual heartbeats inside a measurement window, the watch works out how much the gaps between beats spread, and writes the result to Apple Health as an HRV value. For a usable reading the watch has to sit close enough for the sensor to see your skin reliably, and your wrist should be reasonably still for a few seconds – movement during a measurement degrades the signal.

SDNN – the metric Apple reports

One detail matters for interpretation: Apple Health reports HRV as SDNN, the standard deviation of the intervals between successive heartbeats across the given measurement window. SDNN is one of several common HRV metrics; other systems work with RMSSD instead, which is calculated differently. Both describe the same underlying variation, but with a different formula and on a different scale – which is why values from SDNN and RMSSD systems cannot be compared directly. More on that in SDNN vs. RMSSD: the difference in HRV.

When and how often the watch measures

There is no fixed cadence – and Apple does not publish one. Apple's support page on heart rate states that the watch measures your heart rate throughout the day when you are still, and periodically while you are walking, and explicitly that the time between those measurements varies. For heart rate variability, Apple names the same optical sensor path through the green LEDs that is also used for workouts and Breathe sessions.

There is no number there for how many HRV readings a day produces – and that is exactly how it looks in the Health app: several values on some days, few on others. How still your wrist happens to be plays into it, because movement makes a clean reading harder.

Why the value moves from night to night

Two consecutive nights returning different HRV values is the normal case, not the exception. Several factors work together: the time of day of the individual measurements, the sleep stage a measurement happens to fall into, physical load the day before, alcohol, an infection starting, stress, and simply the natural day-to-day variation of your body. None of these makes a single reading wrong – they only make it clear that a single value says little about a single day.

What a single value does not answer

From one HRV number alone you can barely tell whether a value is "good" or "bad". HRV differs strongly between people – depending on age, disposition and individual physiology – and is therefore practically not comparable between two persons. The value only becomes meaningful against your own readings from the past few weeks: is today's number above or below what has been normal for you in that period? That is the question that can be answered honestly – not whether an absolute number is good.

What it takes to get a usable series

For readings to accumulate at all, the watch has to be on your wrist regularly. For LeibFit that applies to the night in particular, for a plain reason: the app looks at a night window and averages the HRV values that fall inside it. Whether more measurements generally happen at night than during the day is not the deciding question – what decides is that something is in that window at all. A night without the watch is therefore not a less accurate measurement but none at all: for that day the HRV value is missing.

On top of that, every app that wants to read these values – LeibFit included – needs your explicit permission in the privacy settings of Apple Health.

What happens to the raw data afterwards

Apple Watch only delivers HRV as a raw value in Apple Health – it does not interpret it. That is exactly where apps like LeibFit come in: they read the HRV values of the past nights from Apple Health, build a personal norm from them over a longer period, and compare the current value against that instead of looking at it in isolation. It changes nothing about the measurement itself, but it turns a single number into something you can actually use.

Context, not diagnosis

An HRV value on its own detects no illness and makes no diagnosis. It is an indication of whether a measurement lies outside your own norm – orientation for training and recovery, not a medical assessment. If something about your values worries you persistently, a medical practice is the right place to go, not an app.