How to measure HRV: watch, ring or chest strap?

To measure your HRV properly, you need to measure under the same conditions every day. Either at night with a watch, ring or band, or in the morning right after waking up with a chest strap and an app. Which device you use matters less than sticking with it.

At a glance

Measure your HRV either every night with a watch or ring, or every morning right after waking up with a chest strap and an app, but don't mix the two.

Plan for at least five measurement days per week. From three days on, the weekly average is almost as meaningful as with seven.
Only compare against your own values. There is no generally good HRV value.
For night-time measurement, a good watch or ring is enough, as long as you stick with the same device.
For measuring while sitting, during exercise and with an Apple Watch, a chest strap is the better choice. Without any device, you can also use your phone camera while lying down.
Alcohol, an oncoming infection or hard training lower the value noticeably, so it pays to jot down such confounders.

What HRV measures

HRV, heart rate variability, describes how much the intervals between two heartbeats fluctuate. Sometimes there are 950 milliseconds between two beats, sometimes 1,010. This is mainly controlled by the parasympathetic nervous system, the part of your nervous system responsible for rest and recovery.

Devices and apps calculate RMSSD from the fluctuating intervals between heartbeats.

When your recovery system is active, the intervals vary more and HRV goes up. After hard training days, after alcohol or with an oncoming infection, they become more even and the value drops. From day to day, HRV is therefore mainly a recovery marker. In the long run it is also linked to your fitness, and regular training raises it [14]. How to use the value in your training is covered in our guide to HRV-guided training.

Several metrics can be calculated from the same beat-to-beat intervals [15]. Only a few of them matter for your training:

Metric	What it measures	Where you see it	For your training
RMSSD	How much successive beat-to-beat intervals differ from each other. Reacts mainly to the parasympathetic system.	Garmin, Polar, Oura, Whoop, HRV4Training, Kubios	Very important. The standard value in devices and training studies.
lnRMSSD	The same value, log-transformed so it is easier to calculate with	Studies, HRV4Training, your own spreadsheet	Very important if you calculate your normal range yourself
SDNN	The variation of all beat-to-beat intervals in a measurement	Apple Watch	Medium. Useful for your own trend, but not comparable with RMSSD.
HF and LF	Shares of the variation in certain frequency bands	Kubios, analysis software	Low. Strongly dependent on breathing.
LF/HF	The ratio of the two, often shown as "stress balance"	Some apps and chest strap software	Low. Interpreting it as a balance of the nervous systems is disputed [16].
DFA α1	How the pattern of beat-to-beat intervals changes under load	Specialised apps, chest strap only	Important for training intensity, not for the morning measurement

For daily measurement, RMSSD is the number that counts.

Measuring at night or in the morning

There are two ways. The only thing that matters is that you don't mix them, because night-time and morning values are not the same quantity.

Aspect	Night-time measurement	Morning measurement
Device	Watch, ring or band	Chest strap and app, or just your phone
Effort	None, just wear the device at night	One to three minutes every morning
Advantage	The data series breaks off less often	You control the conditions yourself
Disadvantage	You often don't know which part of the night the device analyses	You have to remember it every morning
Cost	Nothing if you already wear a suitable device	Chest strap around €60 to €100, Kubios free, HRV4Training €9.99

If you already wear a Garmin or Polar watch, an Oura ring or a Whoop band, start with night-time measurement. Next to their score, the devices also show you the HRV value in milliseconds, and that is the number you work with. Morning measurement with a chest strap is more accurate and worth it if you already own a strap or use an Apple Watch.

I measure at night with my Polar watch myself. Putting on a chest strap first thing in the morning is too much hassle for me in everyday life. A measurement that runs on its own every night is worth more to me than a more accurate one that I then don't do regularly.

A study with trained triathletes compared how many measurement days per week are needed. A single day showed a hard training phase only about half as clearly as the average of three or more days. From three randomly spread measurement days on, the weekly average was practically as meaningful as with seven [1]. Still plan for five, because in everyday life a day will drop out now and then.

Guide: measuring HRV at night with a watch or ring

With night-time measurement, the device does the work. You only need to keep a few things in mind so the data series stays clean:

Wear your watch, ring or band every night, tight enough for the sensor to touch your skin, but without constricting.
Always use the same wrist or the same finger.
Check the battery in the evening so the measurement doesn't stop in the middle of the night.
In the morning, look at the HRV value in milliseconds, not just the score. The score mixes HRV with heart rate, sleep or breathing, and with Polar, heart rate even has the biggest influence [8]. The table shows where to find the value.
Note confounders such as alcohol, travel or feeling ill. Most apps offer a journal or tags for this.

Device	Score	HRV value in milliseconds
Garmin	HRV status and training readiness	Last night's average and 7-day average under HRV status, plus your baseline range
Polar	Nightly Recharge with ANS charge from −10 to +10	HRV (RMSSD) in the Nightly Recharge details
Oura	Readiness score	Average HRV of the night under Readiness
Whoop	Recovery in percent	HRV in the Recovery view
Apple Watch	No score	SDNN in the Health app

How much a score can swing, I saw in myself in October. In a night with an infection and fever, my ANS charge was at −10, the lowest value on the scale. Two nights later it was at +2.9, and the app said: "You can go all out today!" After a fever, though, no training belongs in the plan for the time being, even if the value looks good again. It shows a calm night, not an infection that has run its course. When you can get back into training is best checked with your doctor if in doubt. The ANS charge is also not a pure HRV value. Polar combines heart rate, HRV and breathing rate from the first hours of sleep and compares them with your average over the last 28 days [8]. With a fever, it is mainly heart rate that goes up, so part of the swing comes from there. Nightly Recharge as a whole then adds the sleep charge on top.

My Polar watch in a night with an infection and fever (left) and two nights later (right). The ANS charge is a mix of heart rate, HRV and breathing rate, not pure HRV. The app is set to German.

The manufacturers analyse different parts of the night. Oura takes the whole night in five-minute segments and averages them [9]. Polar uses roughly the first four hours of sleep [8]. According to Whoop staff, Whoop weights deep sleep and late night phases more heavily [11]. Garmin states a rolling seven-day average, but not which part of the night the measurement covers [10].

This explains why a ring and a watch can show different numbers on the same morning. In a sleep lab study with ECG, RMSSD in REM sleep was around eighteen percent higher than in deep sleep. The values also varied more within one night than between different nights [6].

How much this matters, I calculated from one of my own nights. In May I wore a chest strap that recorded every heartbeat next to my Polar watch. For heart rate, both were almost identical, 57 beats per minute on the watch, 58 on the strap. For HRV, the watch showed 45 ms for the first four hours, the strap 39 ms for the same period. Even bigger was the difference made by the time window alone. From the same strap data, the whole night comes out at 46 ms and the last four hours at 53 ms. That is a single night and not a study, but it shows why you shouldn't compare values from watch, ring and strap with each other, only with yourself.

My own measurement: from the same heartbeats, the time window alone yields 39 to 53 ms. For the first four hours, the Polar watch was 6 ms above the chest strap.

That is no reason to switch devices. As long as you stick with the same device, it measures by the same principle every night.

Guide: measuring HRV in the morning with a chest strap or phone

For the morning measurement you need a chest strap and an app. Two apps have become established for this. Both show you RMSSD directly and work with common Bluetooth chest straps such as the Polar H10 [12, 13].

App	Price	Without a strap it measures via	What stands out
Kubios HRV	Free on the App Store and Google Play	The motion sensor, with the phone lying on your breastbone	Builds in a rest phase of up to one minute by itself [13]
HRV4Training	One-off €9.99 on the App Store, also available for Android (as of October 2026)	The camera, with your finger on the camera and flash	According to the developer, 60 seconds are enough [12], also imports night-time values from Oura and Apple Watch

The morning measurement in five steps. What matters is that you do it the same way every day.

Three things are easy to overlook:

Measure in the same position every day, ideally lying down.
Breathe normally. Deliberately slow or deep breathing artificially pushes HRV up.
If you need to go to the toilet first, do so every day.

Measuring without a chest strap

For camera measurement there is a study with 29 people that found hardly any differences from a chest strap and ECG at rest [5]. However, it was only tested with the iPhone, and the developer of HRV4Training worked on the study. That is good enough to get started, but in the long run a chest strap is more reliable.

How to get the camera measurement right. Place your finger before you start and cover the camera and flash completely. Press lightly, not firmly, and keep your finger and the pressure steady. Measure in a darkened room and take off the phone case [12].

What distorts the value

HRV reacts to everything that keeps your body busy. The table shows how strongly and for how long:

Confounder	What happens to HRV	How long
Alcohol	Drops even with small amounts, the more, the stronger [17]	Mainly the first hours of the night, so your next reading
Infection	Often drops a few days before the first symptoms [18]	Until the illness subsides
Hard training	The recovery system needs longer after intense sessions [21]	Easy up to 24 hours, at threshold 24 to 48, high intensity at least 48 hours
Training late in the evening	Heart rate is raised in the first hours of sleep, HRV over the whole night barely changes [20]	The first hours of the night
Heat in the bedroom	Less recovery overnight at 30 °C than at 22 °C [19]	The night itself
Menstrual cycle	Somewhat lower in the second half of the cycle [22]	The second half of the cycle, every month
Short night	Inconsistent, in one study HRV remained unchanged after shortened sleep [23]	Unclear, still worth noting

A low value after a wedding party says nothing about your training. So jot down briefly what was going on, including things like travel, stress or a heavy late meal. Whether they shift your value, you will see for yourself after a few weeks. Also write down your resting heart rate. It comes from the same measurement and is the more robust of the two values [4].

Pulse wave or heartbeat: what watch, ring and chest strap measure

A chest strap measures the electrical signal of the heart and detects every beat to the millisecond. Watch, ring and phone camera, on the other hand, use light to measure the pulse wave at the wrist or finger. It doesn't always arrive in the same rhythm as the heartbeat, because breathing, movement or cold fingers shift it.

Lying down and at rest, the difference is small, about three milliseconds for RMSSD, around eight when sitting [2]. In a large study with 931 adults, the optical measurement was on average 5.6 milliseconds too low [3]. At night, when you lie still, good devices come close to the ECG, the Oura ring closest of all [4, 7]. More on the limits of optical measurement is in our assessment of how accurately wearables measure.

When a chest strap is worth it

At night, lying down, you can do without one. You reach for a chest strap in these cases:

If you measure in the morning while sitting or standing. That is exactly where the deviation of optical measurement grows [2].
If you measure during exercise, for example to determine your aerobic threshold via HRV. Optical sensors are too inaccurate for that. How it works is explained in our article on the aerobic threshold with DFA α1.
If you use an Apple Watch and want RMSSD. The watch only shows SDNN in the Health app.
If the camera measurement often reports poor signal quality for you, for example because of cold fingers.

What is a good HRV value?

There is no generally good HRV value. Absolute values differ greatly between people, depending on age, genetics, device and measurement method. An RMSSD of 45 milliseconds can be excellent for you and conspicuously low for someone else. Comparing yourself with friends or with tables from the internet is therefore misleading.

Only your own trend is meaningful. After three to four weeks of regular measurement, you know your normal range, meaning how high your values usually are and how much they fluctuate. The manufacturers do it the same way: Garmin only shows HRV status after about three weeks [10], Polar compares with your average over the last 28 days [8].

So pick one device and one app and stick with them. A constant measurement error hardly matters when you compare against yourself, but switching devices does: your comparison baseline then starts from scratch. How far implementations of the same concept can diverge is shown by the study of 206 HRV apps. How little manufacturers reveal about their metrics is covered in our review of the fourteen readiness scores on the market.

How to continue

If you already wear a watch or ring that records HRV at night, you don't need anything new. Otherwise, the free Kubios app with your phone is enough to get started. This is what the first weeks look like:

Period	What you do
Day 1	Choose morning or night-time measurement, set up the device or app, try the morning measurement once
Weeks 1 to 3	Measure on at least five days per week, always the same way. Write down resting heart rate and confounders. Don't change anything in your training yet.
Week 4	Read off your normal range: where are your values most of the time, and how much do they fluctuate? Garmin, Polar and the apps now show it themselves.
From week 5	Build the value into your training and postpone hard sessions when your weekly average drops below your normal range

How to calculate your normal range and when a hard session is better turned into an easy one is explained in our guide to HRV-guided training.

Sources

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